I was in your exact shoes when I purchased my first bike. Everyone always says spend more when that just isn't possible sometimes. Just buy what you can afford and get out there and enjoy. Once you start riding you will find things you like and dislike and can upgrade as budget allows. I suggest ride as many different bikes as you can that are in your budget. I started off looking at the Giant Talon and Specialized Rock Hopper. Don't be afraid to check out some of the smaller companies either. Marin, Rocky Mountain are some less know names but really really good bikes. Just get out there and have fun! Best of luck!