I made a song in fl, but it turned out way too quiet. But when I try to amplify it, it clips, even though it is still very quiet. I don't know how to describe it, it just clips even though it's not loud. Is there any way to amplify it without having it clip?

in audacity(if you have it)I amplify and it doesn't clip..although on one song it did.
but what I did was drag the song way out then amplify then clip it back down to the wanted length

I think you can do that in FL not positive though

Use a compressor.

If the entire track is quiet, I'd suggest you apply FL Multiband Compressor to the master track.  If you don't know how to tweak a mbcomp properly, I would recommend the "Warmify" or any of the mastering presets, but it really depends track to track on how it should be setup.

Also look for spikes or clicks in the audio file , transient needles on edit points. Use a pencil to redraw the wave.
In addition to avinashv compressor mabe look at a limiter 2
 

Use another program.
A friend of mine kept on saying I should use Cubase, I also felt at the time that FL, though 'accessible' still sounded 'muffled' and there was the clipping just when it got good!
Why don't you 'master' it in an Audio Editing program? you could export your low volume original from FL and then beef it up in a proper audio editor.

 'Normalizing' the whole track in your editor could bring up the volume without FLs clipping.

You could go the whole hog and export each individual track you created in FL, import them into an Editor like Audacity where you can 'master/produce' the whole song to a much better sound quality.