Not sure what I would tell myself that I think would have any effect, but...
Be thankful for who you are. Just because you are 6'0" tall weighing only 140 lbs does not mean anything is wrong with you.
In spite of what some of your friends may say, you do not need to be able to bench press 200lbs or more and work out in the gym all the time to look good.
Appreciate the gifts that God gave you. There is a reason why you are as smart as you are. Don't hide it just because others think you are a bit nerdy.
Enjoy the friends that you have rather than trying to find what you think are 'the right friends.'
Go date those girls who was interested in you. Don't expect anything serious to come out of it but do enjoy the time you spend together.
Don't expect your girlfriend to look like that ideal image you have of her in your head. If you do, you will let some perfectly good chicks slip by (and I did).
Remember that girl who you were friends with who you never gave a thought to dating. Date her, I think she likes you even though she does not really show it. (My step-dad actually advised me similarly of this same chick. I ignored him thinking he was wrong. Knowing what I know now, I now think he was right.)
Don't expect anything serious out of any chick you date. Just have fun and live in the moment, the right one will come along.
Reconsider joining the military. The gulf war will not last long and there will be many years of peace before Bush JR takes the healm. If you join, consider the Navy and visit far off places and if you do, consider becoming an officer of aviation. Ignore your dad and step-dad, dad loves the army, step-dad hated the marines.
If you join the military and are not married yet, consider dating asian women, they're HOT!
Go to college, what you cannot afford it? Reread my earlier posts on considering the military.
Oh well, I'm almost 39 now and I cannot live in the past. I can only affect the present. Thanks for the stroll down memory lane.