Going to start mountain biking.

26" AND 29" refer to wheel diameter. 26" is the standard MTB size. 29" are the new hotness. They are very similar in size to 700c road bike wheels (same diameter, much wider).
Frame sizes usually run for like 13-21" in 2" increments, which is often equal to the length of the seat tube from the center of the bottom bracket. I'm a little big for a 17", but a little small for a 19".

If you are going to decide between 29 or 26" wheeled bikes - well 29"ers are usually more expensive, don't have quite the variety of tire and wheel and fork choices that 26" bikes do - just because they've been on the market less time and they are usually higher end bikes. But they are higher end bikes, and often have more ground clearance, and their gearing is longer due to bigger wheel sizes - if your racing downhill, a 55x11 on a 29" tire is a little bit faster than a 55x11 on a 26" tire

Going fast on a trail is hard work! I can hold 20mph+ on a road for over an hour, but put me on a trail, and I die lol.

When you're looking at bikes, I understand not wanting to drop a lot on a high-end mountain bike if you're just starting out, but do consider quality. A low-quality bike will not make your rides enjoyable, and will make it harder to stick with it - I speak from experience. I had a cheapie Columbia bike when I first started mtb several years ago, and it HURT, literally, to ride - my wrists hurt, my butt hurt, the gears wouldn't shift properly. For a little bit more you can get a decent quality mountain bike - if you have any bike speciality stores (local ones are best, but given as I live in Colorado, which is literally the headquarters of the International Mountain Biking Association, I realize not everywhere has local bike stores), go to them and explain your needs and budget.

More important than wheel size is the frame size. Too large of a frame, and you'll get strain/cramps from having your upper body over extended. Too small of a frame and you'll feel cramped. Try different frame sizes to see which ones feel more comfortable - you should be able to comfortably hold the grips without feeling like you're over extending your arms. A bike store can do a setup for you too, to make sure the bike is sized and set up to your body. Around here a setup is like $70, but it will make your ride better by orders of magnitude.

You'll also want to consider if you want a hardtail (no rear shock) or a full suspension (both front and rear shocks). If you live in a place with lots of rocks/ground obstacles, I'd suggest a full suspension to save your hindquarters. The type of bikes you should look at really depends on the terrain you'll be riding.

A niner (29" wheel) allows for easier obstacle clearance, but you suffer on cornering and acceleration. It's really a matter of preference - my husband has a niner (although his main bike is a Trek 26" downhill bike), but I don't. If you're comfortable doing it, try riding both bikes over a curb or speed bump to see how they handle obstacles.

Getting started with mtb depends on your personality - some people, like my husband, just go out and do it, because he has no fear. Me, I fear breaking my leg, so I took a couple mtb starter courses when I first started riding - learning how to corner, how to manage obstacles, body positioning for climbs and descents, stuff like that. If you haven't ridden a bike in quite a while and the trails are at all technical (i.e., lots of obstacles/difficult maneuvering, tough climbs), I'd suggest finding a starter course. Otherwise, pick up a mtb guide for your area, and find easy trails to learn on. If there's a mtb group that does no-drop rides, that's another great way to learn - most mtb'ers are pretty cool (there are some jerks) about newbies, and no-drop rides by definition are filled with people who will help you. Plus you get to make new friends!

Please let me know if you have more questions. My husband's been mountain biking for over about 15 years, and I've been riding for about 5. We'd be happy to help you out!

THANKS !!!!!

I've been looking at different forums about the bike I'm looking at and just like everything else there are mixed reviews. I test rode the bike and it felt comfortable to ride. I would love to have a great quality bike but right now with money and the fact I'm a newbie i would really hate to purchase one to learn on.

I have no fear when it comes to exercising. I have an all or nothing type attitude when it comes to exercising.

You're welcome! And I totally understand about the cost consideration - it's just really important that your bike be fun and comfortable to ride, or else you won't enjoy mtb. Go talk to your local bike shops - just like with cars, last year's model will be drastically reduced, and some places have trade-in programs where you can buy a high-quality used bike that's in good condition. Stores around here also have tent sales, where they'll sell last year's bikes at really reduced prices to make way for the new inventory. This is the end of the season (at least in CO - you might still be able to ride in AR, but we have snow now!), so you might be able to get some good deals too.

You don't need super high quality, but you do want to make sure you have a bike that you like to ride.