Alright, this is the most time consuming of all the achievements. The tutorial for starting a ranked match is listed in the "Here Comes a New Challenger" Achievement. Basically, you just need to win 100 times.
I recommend using Balrog because he is very well rounded as a character. Pick a character you are comfortable with, spend some time in training mode learning how to perform combos, execute special moves, and learn proper distancing and timing. You need to be knowledgeable about your character and his/her matchups if you want to win. Also, alot of players on Xbox Live play as Ryu, Ken, Blanka, or Guile. That means if you pick a character that can deal with Ken or Ryu, you have about a decent chance of playing someone you can handle.
Everything I have learned about Street Fighter 2 has come from Shoryuken.com and its excellent Wikipedia page.