Originally posted by: Fausto1
You might as well just go ahead and get both kinds of bike since that's what you'll ultimately do anyway

. Nothing beats a proper road bike for the pavement and nothing beats a real mountain bike for the trails. The only other semi-viable option is to get a mountain bike and have two sets of wheels for it (not just tires....changing them out is a PITA); the regular set with knobbies for the trails and a lightweight set with narrow, high-pressure slicks for road riding.....also makes a badass commuter setup since you can hit potholes or hop curbs when needed.
As far as a first bike goes, best bang for the buck is to pick up a used one and get your feet wet. It's not really worth the money to buy a low-end bike new when you can get something much nicer that maybe has a few dings here and there for the same price. Most people are pretty anal about keeping their bikes up so it's not like you'll buy it and the thing will break in half on you two weeks later. Spend at least $300 and no more than $500 on a first bike. If you decide you really love cycling you can always go out later and get a really nice bike once you know what you want. Don't forget to budget for a good pair of shorts (yes, they look dorky, but your ass will thank you after an hour or more in the saddle) and a helmet. You're going to need proper cycling shoes as well if you want to ride on the road....the soft soles of sneakers makes for crappy power transfer and sore feet.
Brandwise, stick with any of the big names and you'll be fine. Just make sure you go to a good bike shop to get fitted so you'll know what size frame to get. A poorly fitted bike is a good way to screw up your back or knees. If the employee helping you doesn't at least do a lot of recreational riding, find another store. Only those who spend a lot of time on a bike really know the finer points of proper fit.