Why don't you pose your questions in a cycling forum? I might suggest the recreational cycling forum.
Most people don't read the introductions forum.
Also, we have a 50+ forum that you might enjoy. Join us there.
The 26" that you refer to is in reference to the wheel size, not the bike size. Almost all Mtn bikes have 26" wheels, but the frame sizes generally vary from about 16 inches to 21 inches or thereabouts.
Therefore, you but a bike by the frame size, not necessarily the wheel size.
However, a hybrid bike likely has 700mm wheels (nothing in biking makes sense), i.e., 700x38 or 700 x 28. 700 wheels are larger than 26" wheels, and are about 27 inches.
Frames for hybrids are figured differently than frames for mtn bikes. Oh, my!
You most likely want a hybrid (about $400 and up) or a mtn bike (about $300 and up) or a comfort bike (about $300 and up).
DO NOT, NOT, NOT go to Wal Mart - you will get crap, it will not be assembled properly, it will fail on you, and the components such as the brakes will be way inferior. All the frames will be the same size.
Get the to a good bike shop and look at good used bikes.
Seats is a whole nother issue, about 7 pages long. I will wait on that one.
Check out the other forums!