Very Simple and you will get this over time.
If you see a tank run up close to the right side of the tank (* note is there is a turret gunner on top you must kill him first) it will then show, "Press (Y) to Hijack tank"
Press (Y), then Press the button sequence that comes up on screen (B) followed by tapping (A).
After the sequence you will have control of a tank - A successful Hijack
Now repeat another 9 times
tip* Tanks are awesome for storming a base so keep an eye out for them.