Go to SheldonBrown.com and read the tech articles about internally geared hubs. There's a wealth of information there, and you're better off spending some time to research the topic yourself, rather than have answers fed to you here.

What you're after can be done, but you'll have to pay attention to rear hub spacing--not every hub will fit your frame. Some might work with modification, but others are too wide for your frame. Read up on how to measure over-locknut-dimension (OLD) and then you'll know what size of rear hub you can fit. The front is easy; most hubs are 100mm, whether meant for a fixie or a road bike.

You can get rear cogs (fixed or freewheel) from 12 to at least 20 teeth that will fit on any standard track/fixed gear style of hub. If you're new to this kind of thing, don't try rebuilding the rear wheel to get the hub you want. Buy a complete wheel or have it built for you. Some good sources include Retrogression and Velomine. (Google is your friend.) You can get a complete rear wheel of decent quality starting around $100, or a really good one for $200 or so.