Like the title says, how do you make a note get quiter over it's duration?

I'm using FL Studio by the way.
There should be a sustain level on whatever synth you are using. Look for an amp envelope (it should have attack, sustain, decay and release settings on it). Or you could just automate the volume on whatever track you are using to get something more precise.

I don't use FL, but the concepts are usually universal.
Alright. I don't want it to always get quieter over it's duration, so my only option would be to automate the volume?
Another quick question:
Is there a way to automate volume for multiple instruments at once?
Well I don't actually use FL, so I don't know how to go about doing specific things.

You might be able to copy and paste the same automation curve to the different tracks in the sequencer or you might be able to select several tracks and link it to the one automation curve. I usually do it the first way.