Have a look at Isotropic's FAQ
and scroll down to the section on the leaderboard.
Short version: it creates an estimate of your skill based on your results and the existing estimates of the people you have played. This is expressed as a number representing the average estimate of your skill, plus or minus another number representing the uncertainty. Everyone starts out at 25 ± 25 -- i.e., someone new could be at level 0 or level 50, the system doesn't know. People are ranked on the leaderboard according to their minimum possible rating, so in the case of someone new this is 25 - 25 or 0.
The way to go up in rank is to play lots of games (so that the uncertainty goes down) and win lots of them (so that the skill estimate goes up).