The 100m is a unique race.  You pretty much have to be gifted with natural speed to be very good.  With every other race a person can train and get faster but in the 100m you have to rely on pure inherited speed.

I don't want to discourage you if you think you aren't gifted that much with speed.  There is a lot you can do to get faster, it will just take a ton of hard work.
Judging from that you-tube video of your brother, you must have a lot of athletic ability in the family, so that is definitely an advantage.

You being a girl, training is a little different than for a guy.  As you get into high school and get older your body may betray you physically and there is little you can do.  I have seen countless girls peak as an athlete as an 8th grader freshmen.  Girl's hips start to widen and it totally screws them up.  They get slower and less mobile, but just do your best despite the changes in your body because there is no way to stop it from happening.

If you really want to train and work hard then here is what you need to do:  It is important that you over-train.  You need to run farther distances than 100m to get your body strong and give you the ability to finish a race strong.  I would focus on running a lot of 200s, but you need to mix in some 300m and 400m.  You don't need to run any farther than a 400m to train for the 100m.

It is also important that you start to lift weights.  At your age I'm not so sure your body is ready, but i would definitely start next year.  Lifting weights helps so much.  Most girls don't lift weights, but if you do it will give you a huge advantage.  Just work hard when you are in the weight room and don't go at it lightly.  I would ask your weight lifting coach or track coach for a workout that you can do that is best fitting for you.

Good luck in track, I'm sure you'll do great.  I'm really rooting for you and only wish the best.


-------
http://mlb.mlb.com/team/player.jsp?player_id=405395