I'm a fat dude myself, and as I'm probably a lot older than you (I think they kick people out of high school at 20, so I'm safe in saying that), let me impart on you a little of my wisdom:
You're screwed. Either start exercising or get yourself a good antidepressant. If you go for the latter, try to convince your doctor that you have anxiety problems so he'll give you a good benzo that you can get high on occasionally to dull the pain that is your miserable existence.
I'd suggest the former, however. At 150 (is that even that heavy for a high schooler?), it's a lot easier to exercise than at my weight. For you, a little exercise will probably make you tired. For me, it causes tremendous amounts of excruciating, agonizing pain. Take your pick.
As for your lady friend, just don't be a weenie about it. Be confident (but not too much). Get friendly with her and then ask her if she wants to go to a movie or just hang out or something else casual. If she has a good time, ask her if she wants to hang out again sometime. Then if it doesn't naturally progress to holding hands and kissing in the hallways and giggling and all that disgusting crap after a while, just tell her you have a crush on her (but not that you're madly in love with her and you have a hair doll of her, girls tend to find that creepy). If she's still not into you then, tell her to get lost and spread rumors that she gave you the clap. Or just stay friends, whatever. Just don't dwell on her and obsess over her, it'll make you miserable and it's really unnecessary when you're that young. Wait until you're my age to obsess over girls, you'll be able to appreciate the misery much better, like tasting a fine wine.
Everybody goes through what you're going through, don't worry about it. It's just how you deal with it that matters. You can either realize you're young and it's ok to take chances, and that if you fail that that's ok too, or be timid and passive and obsess and dwell over things and become a miserable old ******* like me.
But seriously, exercise. Christ, if I could go back in time, the first thing I'd do is tell my younger self to exercise. He probably wouldn't listen though, because I'm a stubborn little prick, but eh, that's moot.