First round starts, use your pistol to kill to kill a guy, run over and pick up whatever weapon he's using. Don't bother buying any guns until wave five. If you need to, run to the gun armory and refill your ammo.
You get three armories in the first three waves, guns, equipment and I forget the last.
Really utilize the (argh I can't remember what it's called) the kill-streak armory? Anyhow, you get this armory in round 3. Delta squad is decent, but the Riot Shield Squad lasts longer. I can't remember when you unlock them, but they're useful. Predator missiles are great, make sure you're in cover when you use them. I don't really use the airstrike, don't have a comment on it.
Helicopters, in the earlier rounds, can be taken down by SMG or whatever automatic weapon you have. Empty about two mags into it and it's gone. Later you'll need LMGs or predator missiles to take them down.
Juggernauts, in the earlier rounds, can be killed easily by C4. Just find the place where the juggernaut is being dropped, lay down all five C4 explosives, and once he hits the ground, detonate. Later you'll need LMGs or predator/airstrike missiles to kill them.
Don't bother with buying the body armor, waste of money in my opinion. I never use it. Although self-revive is amazing. But if you find yourself having to buy it every wave..... good luck.
Sentry guns are awesome, just make sure you stay nearby their location because at the higher rounds the enemies will take them out.
Claymores are great, but make sure you place them precisely because enemies will shoot them if they see them. C4 is awesome for clearing out a group of enemies all clumped up.
Lastly, conserve your money. Spend it wisely.
Hopefully I'm not forgetting anything...I play a lot of survival, just a few tips that should help get you started. Enjoy playing it!
. Will use your tips !! 
