Hi,
There's two main ways to do this, but there's a bit more to it than that.
Your track levels can be measured by peak which is very relevant with regard to the limits of digital media, but less relevant in terms of how loud the mix sounds.
They can also be measure by average (RMS) which relates much more to how loud the mix sounds to you.
On top of that, though, frequency content and balance will affect how loud a track sounds to you.
All of these things need to be considered.
Basically, it's entirely possible to have your tracks all 'the same' volume and still wonder why one of them sounds quieter than the rest.
The two ways I mentioned..If you have working sessions available to you you can use volume automation to adjust the levels of individual tracks at different points in the mix.
If you're not familiar with that concept, have a quick google/youtube. I don't know if FL Studio has the option. Hopefully someone else will.
Automation is usually represented as a line along your track (Wave view) from start to finish. You can add points and increase/decrease the level of that line.
Professional recording suites will usually let you automate just about anything.
Volume, pan, effect levels and parameters. etc.
The other way, if you don't have access to a session, is simply to do exactly the same thing on your master track.
It won't be as precise but it might be OK.
The difference is just like the difference between fading a song out by pulling down the master fader, or fading a song out by gradually pulling down all the individual faders, perhaps at slightly different times.
Hope that's useful to you, and welcome to HR.
But maybe, 'newbie + 'first time 'just starting out', How about level adjustments (for the first). Then include comp/limiters if needed.
