get a bike from a bike shop. these are a few of the main reasons why:

- they will put it together correctly because they know how, it's what they do. can you build a bicycle? (based off you question, i'll guess no. i don't either by the way.) the guy working at walmart/target/sports authority/etc knows as much or less than you do about building a bike.
- usually you get a year or 3 or 6 months of free basic maintenance, or at least a free tuning check up after X months. the point is, when something is wrong, you can go back to the bike shop and they can and will fix it.
- the bicycles at a local bike shop are higher quality. the cheapest bike at a bike shop is better than the most expensive bike at walmart/etc. that means they perform better and last longer and in the long run are cheaper because the parts aren't breaking and you don't need to replace parts all the time.
- fit is the most important part of a bicycle. a bicycle shop will (usually) make sure you get a bike that fits you.
- cycling is a sub culture, a way of life, a fraternity, whatever. you don't have to join it, but if you ride a bike, you are in it. a bike shop is sort of like the local frat house/teamster hangout, whatever. you make friends there and are part of a "club."

so if money is an issue, (you can get a decent entry level bike for a few hundred.) and you can wait, save up the money and get a bike at the bike shop. ESPECIALLY if you intend on riding the bike on dirt trails.

trek is good, really, any bike sold at a bike shop is meant to be ridden and will be reliable and good. again, fit is everything. try all the brands of bikes and get one that fits you well. hardtails are great bikes and are relatively inexpensive.

good luck. rock on.

oh, and stay off the sidewalk. but that is another topic entirely.