Well, we need to know a couple of things about your goals.
To start out..its just to get me active, physically. Its been a couple years since I have been on a bike..nothing great. But I also weighed less then too. I'd like something for exercise..to drop weight..then I'll take it from there. 
Mostly we need to know if you are going to be on a bike path/roads, or mainly on trails.
I guess a liitle of everything. I live at an elevation of about 7600ft. Mountain terrain just out of town..hill's in town. Roads in town, trails in the mountains. I will start out easy and save the trails for farther down the road though I'm thinking.
Also, do you eventually plan to commute to work or use your bike for errands?
Errands probable. Work, no. Its about 50 miles one way to my workplace.
Any physical limitations such as bad back or shoulders that might influence your riding posture?
Just bad knee's, from too many days of highschool football.
What is the terrain like where you live? Flat, or hilly?
mostly somewhat of an incline or full fledged hills.
Do you have a target budget?
something reasonable..maybe around $500?
Are you mechanically inclined and have some basic tools?
yes
Thanks again!