With some songs, I can sing and play they song at the same time, but with other songs(Like Boss D.J. by sublime) It's quite hard for me to sing and play at the same time.
Practice. i have the same problem. Singin in a different rhythym and tone than your guitar messes me up the most...
I'd recommend learning "Hey, Hey" by Dispatch. Thats a good one.
Like Angel Of Sorrow said it's all about practice. You have to get to where you can play the rhythm without having to think about it, so your mind is free to deal with the lyrics. Just play the rhythm till yur sick of it, then play it somemore, slowly try to incorporate the lyrics, and try humming the lyric lines as you practice if you're still having problems.
Singin in a different rhythym and tone than your guitar messes me up the most...
If i could solve that very problem by any means i would do it. I can play VERY simple chord based songs and sing fine, but anything else i completely screw up.
By any means, Im no singer, But when im alone messing around or even jaming with some of the boys, What I do is learn the rhythem until it is downpat to the point where you played it to yourself to death pretty much, then try talking to someone while maintaining the rythem, Listen to the song for a few times, then start singing while your chords (I asume you play chords and not lead), and it should just come with practice.