Does anyone know a way to level up fast on call of Duty 4?

Use guns until you get the challenges out of the way then move onto other guns to keep building the points up. It's a 1000 xp for the first 150 kills with guns. Always check your challenges and try to go for those easy ones its usually at least 250 xp for completing those challenges. Kills are an easy ten points so try to learn the maps and know how to get the drops on people. No real "easy" way to level up just a lot of playing and learning the ropes on how to get away with a lot of kills and minimal deaths.

Just keep playing and you'll get better, yea once you've done one achievement with one gun move onto another one.