oh my goodness, you don't need to lose any weight! but i hear you on the whole "wearing a bikini" bit. sounds like you just want toned muslces...i think pretty much everyone's that way.
for your stomach, try doing sets of sit-ups everyday. start with two sets of 30, and then gradually increase your repetitions
thighs: squats, running, biking, etc...just work on these things and over time, they should help
arms: pushups (they're really not that bad once you get good at them). if you have weights, google something like "arm weight exercises" and you'll find stuff to do
armpits: well, i'm not exactly sure what you mean by armpits...but try to do exercises that'll work your shoulders, upper back, and chest. basically, these exercises will most likely alsowork your arms
well good luck. just keep up with exercising. try to do a half hour each day if you can. it may take a little time, but you'll see improvement. listen to your ipod while exercising, it helps a lot. exercise with a friend. set goals. good luck!