Which is the best for a beginner to start with ?
Halo
Call of Duty
Battlefield
Gears of War
or some other .
Would like to do some easy missions and play online .
Halo will be your best bet. It has a lot of different difficulty levels that scale well with experience. Also, if you are playing with a buddy, there is a nifty feature that will let you re-spawn as long as you don't both die.