A better microphone and preamp is a good place to start. Essentially if the initial recording is of a certain quality, it will never really get any better. You can run a million noise reduction FX but bottom line is hardware.
That being said, Dynamic Compression and the right Equalising can do wonders. The problem with compressing a bad original signal though is, all that white noise in the background will actually be increased in volume! hence why it's important to capture a nice clean signal originally.
You might want to try googling equalizing, its a bit of an art and highly dependant on the individuals voice. Less is generally better, and cutting frequencies as opposed to boosting them is prefered.
Compression will just even out the signal in general too, so it is worth looking into if your vocal technique tends to go very loud then very soft.