I never have problem writing riffs,but im just unable to find a second part or whatever to lead the riff into making a song.how do you create a song if u find a riff first?
Have you written two riffs before?  answer: yes.  Have you tried putting them together? Answer...
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Non smart ass answer: (well, still kind of smart ass) play your riff then play it backwards.  Bam, you got a song.  You don't even want to know how many punk songs I wrote that way years ago.  Good little exercise to get the juices flowing and keep you from overthinking the different parts that construct a song.  Also, if I've learned anything from Doom Metal, there is nothing that says a song can't be one riff.  
"I definitely don’t write all my music in a blackout, like I used to, although I did come up with some good stuff in a blackout."
-Matt Fucking Pike
Yes, as Badluckpalms says, play it backward to give you a clue, then add a slight variation and return to key note. That should give you a basic idea. Often the secong riff is less complex and contains more of the same notes than the initial riff, so try to do a simplified version of the first riff. Also. think about Question for the first and Answer for the second.
I think continuing an idea is the most difficult part of songwriting, and I'm pretty sure many people struggle with it.

There is really no answer to it. You just need to try different things. Listen to the first idea a couple of times - what would you like to hear next? Do you hear any sounds in your head? Sometimes you just need to start with something. Ideas can always be developed.

You could also change the original idea a bit. I mean, for example the last repeat of the riff could be a bit different and it could be used as a transition to lead to the next part of the song.

Some songs only have one riff/chord progression in them (good examples would be "Smells Like Teen Spirit", "Whole Lotta Love", "Get Lucky" and "Treasure"). So you could try writing something like that. That of course requires you to use more contrasts to keep it interesting and the melody needs to be strong.

Also, if the main riff is rhythmically complex, the next section could use long power chords or something like that. And if the main riff is loud, the next part could be quieter. And if the main riff is centered around guitar, maybe the next part could use some other instrument in the lead role and have the guitar more in the background.

But yeah, if you have a hard time coming up with something, try to think of a feeling that you are after. What do you want the next section to be like? If you know what you are after, it will be easier to come up with an idea.
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