Don't know what you'd need to do to go fixie, but you can turn just about anything into a singlespeed for next to nothing. If it's a triple chainset, you'll need to get some shorter chainring bolts and you'll need to find some old cassette spacers, but that's all you'd have to have that you wouldn't already have on the bike. This is an absolutely vast thread from another forum, but you can find out everything you need to know about making a ghetto singlespeed in the first few pages. 
Handy chainline instructions and diagram.
Drink coffee and do stupid things faster with more energy.