There are two ways: You can normalize your tracks. Normalization figures out how much louder it can make the loudest part of a track without it clipping, then make all the rest of the track the same amount louder.

Or you can compress your tracks. Compression takes the loudest parts of your track and makes them a little quieter, and takes the quietest part of your track and makes them a little louder - then boosts the entire track to as loud as it can be without clipping.

Compression is more aggressive than normalization and you can really make your track louder... but it's a black art! You are in danger of damaging the dynamics of your track: it's louder, but not as "punchy". Pop songs are compressed to fuck, and resulted in the loudness wars.

However, good compression is still really impressive - and you can find some good audio plugins (if you use a DAW) with good presets to get some impressive results.

Just always be sure to A/B test the changes to make sure you're not losing a bit of the magic!