Well, it's mostly a matter of practice, try starting with simple songs, and see how the vocals relate to the guitar playing, then go slowly building towards harder songs.
My guess is you've only been playing for a short while. It will come, you just need more practice.
Start with easy songs and whisper the words. and build till you've got it.
the better you know a song when you play it, and the better you know the words, the easier it is to play and sing the song. when you have to think to remember the words or how the song is played, it can be impossible to do both.

so as the others say, practice is a big part of it. when i was playing in a cover band, i had a cd of all the newer songs so i could listen in the car to learn the words and changes better.
Just practice the song until you are comfortable playing it then start singing. Once you have the song down you can add the vocal.

It's just practice mainly.
What helped me was forcing myself to focus. I basically decided I wanted to be able to play and sing at least one song. So, I focused on that one song. I tried to go over the lyrics in my head without playing so I wouldn't have to think about them. The other thing that helped me was just strumming each chord once while singing to just work on the changes and then working up to a more complicated strumming pattern. Good luck
I would endorse all these answers, pretty much covers everything I had thought of.

One thing I would add that helped me a lot is just slow the song down while practicing doing the guitar and singing so that you actually have time to think of everything you're doing (strumming or finger picking, fretting, getting the right word and note.) For myself when I had just started playing and singing with more complicated finger picking or strumming patterns I found that doing it slow first and understanding the timing of each seperate part of the whole made it a piece of cake to speed up.

Best of luck!
UG 83
An easy-ish song to learn is Hard Days Night by the beatles, im using it to practise with and it's quite good, good song too
Also another I like to do myself I tie down specific words as sort of markers. If you have a repeating progression try and remember what words are being said at each note or whenever the progression repeats. Its easier with simple songs in a 4/4 timing. The more complicated songs have words that don't match the rhythm of the song. Worry about those later.
This is what I wrote in a thread a while ago when there were some people concerned that you had to be gifted to sing and play. It only takes time, like everything else in the world.

Myth: Only naturally gifted people can sing and play at the same time.

Truth: With patience, nearly everyone can sing and play simultaneously.

Explanation: For nearly everyone, starting off trying to sing and play sucks. It's an obnoxious cycle of strumming with the words or singing with the strum pattern and it just sounds like a mess. That's normal. It's exceedingly rare to find someone who just gets it right off the bat. Here are some tips to get you going if you're motivated to add the singing dimension to your music:

- Start slow. Try putting the guitar aside and just tapping your foot while singing an easy tune. It's a trivial exercise for some and hard as hell for others. Either is OK. Keep going until you can keep a steady beat and sing the song accurately.

- Learn a song well before you try to sing with it. This is a big one. If you still have trouble keeping a steady strumming pattern going while transitioning between chords, adding vocals will be really difficult, nigh impossible. If you've only been playing for a few weeks, give yourself a little more time. It will only make the learning process smoother.

- Start with a (really) simple song. There may be a thread for these somewhere, but I haven't looked. The idea is to pick a song with very basic chords (for example G, C, and D in the song "Stir it Up" by Bob Marley).

- Once you get a song, it sometimes helps to dumb down the progression a bit. For instance, strum quarter notes using only down-strokes. Try to hold a steady beat while you sing along and keep the chord changes smooth. This wont be easy. Give it time. The fruits of your labor will come eventually.

- If you've got that down, your essentially there. Next is to mix up the strumming a bit (say to the common DDUUDU pattern). From here, it's just time and practice.

Many players fear that just because they get the technique down, it doesn't mean they sound good because they have a "bad" voice. Unfortunately, "good" and "bad" preferences are just part of the game. I would bet that a couple people thought Louis Armstrong had a pretty crappy voice, but look at where he got. The trick is confidence and character. If you insist that you sound terrible, you most likely will.

Here are a couple bits of advice from my experience (please note that I am no professional singer, nor have I had any professional training):

-Sing loud, sing proud, and sing all the time. Sing in the car, sing in the shower, sing everywhere that you can. The more you practice, the better you'll get.

-Sing in front of people (eventually). Your nerves will get to you, but performing in front of a close friend or two will always get you some useful input. And hey, even if they think you sound bad, you'll still get respect for having the cojones to do it.

-Explore your vocal range. See how high and how low you can go, stretch your boundaries (safely, so as not to injure yourself), and find artists that have a similar range to you to practice with. There are great singers with both high and low voices.

-There are many more tips out there and I'm sure most readers are fairly proficient with Google and whatnot. Look around if you'd like some more advice.

Just for fun, here are some successful guitarists that have interesting (meaning bad, to some people) voices:

Paolo Nutini
Billy Corgan
Brody Dalle
Billy Idol
Bob Dylan
Jack White
Julian Casablancas

And so on...

And an informal side note on snagging ladies:

For the male readers who are concerned with singing to impress ladies, the most important thing to know is that girls usually aren't thick-headed, meaning that they probably have an idea that you're trying to woo them (think of the Jazz-flute scene from Anchorman). That can work to your advantage. Even if you think your voice is bad, sing like you really mean it. Confidence is huge. Slipped notes and playing stumbles don't matter if the woman knows you really are giving it your all.
Last edited by GC Shred Off at Sep 24, 2010,
I JUST taught myself how to sing/play at the same time after weeks of googling "how to sing/play guitar at the same time" and not finding anything useful...
And I'm pretty decent already!

**one tip: Chord based songs are THE BEST and EASIEST and FUN-EST to start learning with.
I learned how to sing and play with "Good Riddance (Time of your Life)" by GreenDay.

If you're learning a song that goes:
G chord
C chord
D chord

and the lyrics are:

"Another turning point a fork stuck in the road".

You neeeeeed to listen to the song, and

*Write down where you can hear that a different chord has come up in the song
(it doesnt even matter if you know what the chord is or not).
*This might look something like:

Chord Chord Chord
Another turning point a fork stuck in the road.

If you can find tabs/chords/sheet music to the song/you know that the song starts:
G chord
C chord
D chord
then, you can assume that:

Another turning point, a fork stuck in the road.

Now, if you've listened to the song, you know that there is more than one strum for each chord.
BUT, if you want to sing the song while playing it, you need to:
1. Focus on singing the song first.
-sing the song, and strum ONLY ONCE where you see a new chord.
2. Figure out the strum pattern*
THIS is what makes learning how to sing/play HARD.

Once you've got singing the song and stumming only ONCE where every chord is in the lyrics, you'll want to be able to strum along with the song.
*trying to do the EXACT strum pattern you hear in the song is HARD/IMPOSSIBLE/VERY FRUSTRATING depending on the song....so, its recommended that you figure out a similar pattern.

To figure out the strum pattern:
1. Figure out where each chord changes to a new one and apply it to the lyrics.
2. Usually wherever you see a new chord it can be assumed its a down-strum (V).

V (G) V (C) V (D)
Another turning point a fork stuck in the road.

3. NOW, break down the lyrics into SYLLABLES.

Write down wherever you can hear that a chord is distinctly being strummed (these are usually downstrums too) on a syllable.

A-no-ther tur-ning point, a fork stuck in the road.

Slooooowly play and sing the lyrics syllable-by-syllable with a downstrum on each syllable (in this case).

4. Count how many downstrums you found. Then, listen to the song again and count how many strums there are that you can't tell are down/up strums.
*(Just to help you out:
-"Time of your life" is in 4/4 time
-Each strum is an eigth note.)*

5. The strums that you can't tell are down strums are *GASP* Up strums(^)!!!

6. Apply the up strums to the syllable-by-syllable lyrics.