With the tank, you have to get close to the turret and then press 'E'. After this, when the tank driver pops his head out, press 'F'. Once you've slapped the pistol out of his hand, you need to keep tapping the SPACE bar... This will allow you to struggle with him and eventually you grab the grenade, give him what's sometimes referred to as a 'Glasgow Kiss' and your character then pulls the pin from the grenade and drops it inside the tank. You're then free to drive the tank around.
That's all assuming you're playing the PC version. If it's a console you're using, I'm afraid I've no idea...
Hope that helps