Win a side, defend base, win the other side.
It's really that simple, but many people don't do it. Most maps, there are 2 sides or corridors or flanks. Once you pick one, count tanks. If you have more than they do, you have an overmatch. Be aggressive and help your team win that side. If you have less, defend it.
If you win the side you chose, don't push to the enemy cap, instead go back and defend yours. If it doesn't need defending or you have defended it, then go help win the other side.
Disclaimer: This is general advice. There are situations when you should cap, but take a mental note of how many people die pushing to or sitting on the enemy cap in your games. You will notice that it is a bad idea more often than not.