Tanks are your mortal nemesis in this game as a ground troop. Basically, if a tank shows up, run as quickly as possible to grab a panzershreck and take it out.
The tank not only boasts an incredibly powerful cannon, but also a secondary machine gun that will gun you down as fast as you can make it up to the tank. Don't bother shooting at the tank with anything other than the rocket launcher - nothing else will put a dent in it (with the exception of levels where you are in a tank yourself).
For more vehicle warfare, check out the Vehicles Challenge that's available within the game. To learn about tips on using buildings, grenades, mortar, and other weapons, continue reading the second page that will guide you through each element of Call of Duty 5.