You might not know anything about bike mechanics now, but I'll give you the benefit of the doubt that you can teach yourself. As a (more or less) rocket scientist, I'll tell you you don't have to be one to figure out how to work on a bike.

look at your local library for "Zinn and the art of Road Bike Maintenance" read it, wrench a few things on your bike and you'll figure it out.

if the bike's older (like mine) you'll find you need 4-5 tools for most work. a 5/8 wrench, a 9/16 wrench, and 8 and 10 mm wrenches. you might find a few allen screws that need allen wrenches (allen wrenches are inverse socket wrenches if you don't already know) 6mm is the one I seem to use most. a tire lever is also handy, but my experience is the ones that come on multi-tools wear out quickly unless you're good at changing tires.

I'd say buy a good multitool for $15-25 (which gets you all but the 2 big wrenches) but count that outside the cost of the bike, why? because it'll be your friend every day you ride.

sheldon brown's site has almost everything you'd want to know about fixed gear. I haven't done my conversion yet, so I can't really throw a cost at you. you'll probably edge closer to $200 is my guess.