Need help buying a bike...

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My girlfriend lives about 3 and a half miles away from me and I've been taking the bus everyday to go see her. Recently I came up with the idea of cycling instead of taking the bus, for some exercise and to save money. I have some questions before I go and buy a bike :

1) What size is best for me? I'm 15 years old, about 5 foot 9 inches and a little overweight (part of the reason I'm cycling is to rectify that :p).
2) What kind of bike is right for me? I will be cycling mainly on paths and streets, nothing too bumpy (no large bumpy hills and stuff) if that matters.
3) Can I cycle on the road? Do I need a special licence or anything?
4) How much do you think a decent bike will cost me?

As you can tell, I am a complete n00b to this cycling stuff so don't get too technical on me :)

I like my Next Monterey Bay from Wal-Mart. The 3-speed hub is reliable and maintenance-free. It costs a bit more than an ultra-low-end mtb from a department store but is still pretty cheap. You may have to go to Walmart.com to find one. Not all stores stock them.

The braking system is rain and snow proof but you would have to add fenders to ride in the rain. However, it does have a chain guard so it won't mess your pants leg up.

I would probably walk (power walk) :D, better exercise and cheaper than cycling. Take about 40-45min.

The money you save on the bike or bus could be used on a soda or ice cream :p.