When buying your first bike go for the best FRAME you can afford. The Frame is the most important part of a bike. The best frames generally available are Cannondale and Trek (I ride a Cannondale). Giant makes a very good bike (For a Chinese bike, almost as good as a Trek or Cannondale).
Once you buy a frame you can change components one at a time as your riding improves. New Wheels, new gears etc. For Example I purchased my bike as a frame only offer from Cannondale (i.e. only the Frame not components). I than striped off the junk components off my old bike and rode as I slowly replaced the Components with Better Components (Especially as I learned about the better components out there).
Now as to the actual frame I generally would recommend a hybrid that has 700cc wheels and tires (Unless you are smaller than 5'4" than you should look at 26 inch wheels). I like the 700s for they tend to be narrower than tires for 26 inch bikes. 700cc bike wheels get they name from being the most popular wheel size for racing bikes and since the most famous bike race is the Tour de France, the French term is used.
The 26 inch wheel is descendant from the old Schwinn bikes of the 1950s. When Mountain biking started in the 1980s the first bikes were converted old Schwinn's using 26 inch tires. Since most bikes using 700cc wheels could NOT handle the wider tires of off-road racing, the 26 inch tire just took over Mountain biking.
Now unless you plan to go riding up and down over perfectly good hills, I would recommend a bike with 700cc wheels. It will get you a good start.
If you are 5'4" or smaller you may have to stay with 26 inch wheels (The wheels are just that much smaller that it is easier to design a smaller bike with 26 inch wheels than with 700cc wheels). If you do opt for 26 inch wheels one of the first thing I would do is to look into replacing the tires with narrower "slicks" i.e. tires with the knobs. To sell bikes mountain bikes are always sold with the widest knobbiest tires available. These are NOT the best tires for Highway use. A narrower tire that can be inflated to higher pressure is a much better tire for highway use. I do NOT like going under an 1 1/4 (or 35mm) for that seems to be the best balance between minimizing pedaling and still be able to stop. Remember the more tire you have on the road the harder it is to pedal, but easier to stop. 35mm seems to be a good balance (It is rare to find a 26 inch tire narrower than 1 1/2 but a lot of 700cc come in 35mm widths).
For most people that they need to know when buying a bike are the following:
1. Does it fit you? Basically the bike top rail should be low enough so that you can straddle it while standing, but the seat should be tall enough that when you are peddling your legs completely stretch out. A lot of people who bike who complain of knee problems have they seat to low, they legs do not full stretch out so instead of "walking" on your bike you are crouching while peddling and this is bad for your knee. Now a lot of people who bike want to be able to touch the ground while on their seat, I am sorry but that is defeating your knees. If you look at the professional bikes they tend to start they bikes while straddling the top bar and then mount their bikes after it has started. One technique I have often used is to use a street curb for my feet while I mount the seat and then go. A curb seems to be the same height as my peddles so it is an easy way for beginners to learn the techniques of starting their bikes.
2. If you are riding on Asphalt or Concrete (as opposed to dirt Roads) you do NOT need anything wider than a 35mm tire. The narrow the tire the easier to peddle AND THE GREATER TIRE PRESSURE THE TIRE CAN WITHSTAND. Higher Tire Pressure also makes it easier to peddle by reducing "Roll Resistance" to the tire.
3. I question the need for any suspension on a bike unless you are going cross country. The purpose of Suspension on Mountain bikes fall into two categories, the Front Suspension which is to help the rider retain control of his bike if he hits a real deep pothole and the Rear Suspension which is to keep the tire on the ground when do to going cross country the bike jumps ups and down (Which picks up the rear wheels so that it is no longer in contact with the road). One bikes intended for paved roads, the rear suspension will NOT help you are all, and 99% of the time a front suspension will not help you (and most of the time you will be able to go around any pothole in the road, or prepare yourself for the hit if you have to hit a pothole. On road use I find suspension a waste of money.
4. Conventional hand brakes are good enough, disk brakes are for people who want to go down perfectly good mountain sides.
5. The big push for more and more gears is driven by racing nuts who want that little edge. Now once you have biked for a while replacing the chain and "transmission" may be one of the first things you will do. I personal story, I had several bikes in my life, mostly "Ten Speeds". The components were "junk" so all I ever used them for was to find the best general gear for me and stayed in that gear. When I upgraded to Shimano XT, I was able to shift quickly and as I needed. These were easier and quicker than anything I had ever used before and finally learned why people paid good money for better components. in many ways that is the difference between a Toy store and a bike store when it comes to selling bike. A bike store is selling bikes that are intended to be used as a mean of transportation NOT as a gift for a kid who may or may not want it.
My Advice is to get a Giant if you can not afford an Cannondale, and than slowly upgrade the components over the next few years as you get more and more into biking.
Here are some sites with articles and advice on bicycles and biking:
http://www.sheldonbrown.com/articles.htmlhttp://www.sheldonbrown.com/brandt/index.htmlPeter White has a Good Article on Fitting a bike:
http://www.peterwhitecycles.com/