Pointers for buying a bike...
1. BUY IT FROM A BIKE STORE. Walmart and target may sell a ridable bike, but bike stores will fit you to a bike. Plus, most will give you the first checkup free. That is important considering the chain will stretch some and the derailers will need to be adjusted. They will answer your questions with correct answers other than it looks good.
2. If you sole purpose is to lose weight, you still need to be comfortable. Ask to ride it around to make sure the seat wont poke a new a-hole. Your bum will be a little sore after a the first we ek or so of riding, but it gets used to it (that sounds bad). Do not go out and get a titanium or carbon fiber frame. MUCHO DINERO. Much mucho dinero. An aluminum frame is fine to start off with. Steel is okay too, but it can rust.
3. Suspension bikes may have a softer ride, but it takes more power to go up hills. Notice when you try one out, you will "bob" a little bit with this. Hard tails do not have it, but hard tails(no shocks) have a harder ride in comparison. Suspension bikes do have knack to weigh a little more. They also cost more.
4. Realize what kind of surface you are going to ride on. I use a cross trainer wheel, road and off road treads. using big knobby tires sucks on roads since it has way to much traction. Then again, slicks are horrible on dirt. I spend about 70% on roads mostly going to class and 30% on trails on the weekend.
5. If you ask what the best bike out there, you are going to get a different response no matter who you ask. A $200-$300 bike is really going to be the same anywhere you go, but with different paint. I personally like Trek just cause that is what i have. It would probably different if i had walked into a schwinn store.
6. 24 or 18 or 21 gears. Doesnt matter. You are more likely to use the middle range in your daily riding.
7. A HELMET is a must.
8. If you plan on riding at night, you must have a white light for the front and a red reflector that can be seen from 500ft for the back. This is the law. More so for safety.
9. Bicycles have to obey the same rules as automobiles do. Weaving around cars will get you killed as you will be unpredictable
10. Do not ride in the gutter. That is where you will get the most flats.
11. Get some sort of hydration. water bottle, camelbak etc..
12. If you break something or something squeaks, take it to the store. Dont touch the derailers unless you know what you are doing. Usually it is a bolt that wasnt torqued correctly and will be fixed for free.
13. Get a good lubricant, like tri-flow and ask how what kind of maintenance you need to do.
Anyways, sorry i posted so much...Im currently planning a bike trip in New Mexico for this summer.