There are a lot of answers to this question, the most simple is turn up the sliders a bit at the bottom of each channel. This may cause stuff to peak out (clip), in which case you need to either stick a limiter (a compressor with the compression ratio turned up to infinity) or add some compression. Equivalently you can turn up your master volume slider. If this is clipping then add compression to the master bus. The less dynamic range (and headroom) you occupy the louder you can get it.

You might want to try some mastering software like wavelab. I use it with Waves Native Gold plugins and it is very good !!!