Plutonium

Soldato_DG
I had a tough time when I first started back in 2008, round 3 on Nacht scared me so much as a kid.
Learn to train zombies around. (run around in wide circles to abuse the zombies' AI)
Get a good feeling of map layouts and where the best spots to run trains around.
Maximize points in early rounds. (you earn 10pts per bullet per zombie so penetrating multiple zombies nets more points per bullet, kills net 50 points, headshot kills net 100pts instead, and knife kills net 130pts instead)
Find what weapons feel most comfy to use, but learn to roll with and have fun with the absurd weapons and their upgrades.
For BO2 maps, you'll need to learn part placements.
And finally, join some randoms and friends and just mess around, it's the best way to have a good time.