There are a total of 50 ranks available in Halo: Reach. The first is Recruit and the last rank is Inheritor.

Whats the fastest way to rank up in Halo Reach?

So really the best way to level up fast is to play as often as you can. Focus on quick games like SWAT, if they are available. Firefight, online multiplayer, and the campaign all offer plenty of XP rewards for playing, and will give more if you do well. Try to get a lot of kills or score the most points for your team.