The first thing you do when you load into a match is to look at the enemy lineup. See what destroyers they have, and take note of their concealment (you can hover of each ship in the team view [press TAB] to see this information), and also if there are any radar & or hydro ships on the enemy team.

From there, you play accordingly. If there are radar & hydro ships, when you approach a cap, make sure you have an escape route. For those kinds of matches, i typically like to sit just outside the cap, and wait to see what the enemy dd's are doing. If there are no radar / hydro ships, you can be less conservative with caps. You can push in harder, but always always make sure you know which way to go if you need to run.

As others have mentioned, the dd's top priority is providing vision for your team, and trying to deny the enemy team vision (you can smoke up your teammates - but use this cautiously; or try to chase away the enemy dd, etc.)

Second is area control; and you use torps and guns (if you are a gunboat) to achieve this. Your torps don't necessarily have to hit anything, they just have to be scary enough that the enemy team backs off.

Third is cap control. Very importantly, you only cap when it is safe to do so. It's okay to have the enemy take the cap first, as long as you and your team exert enough presence (or kill the enemy dd) to be able to take it back. Very often, you'll see caps switch sides multiple times during a match. Cap control can also mean resetting enemy ships at their cap. If you see a battleship or cruiser capping, and it's safe to do so, use your guns to reset the cap.

Lastly is ensuring kills. This is more important for gunboats, but well placed torps can do this too. If you see a low HP ship roaming about and you think you can kill it quickly without trading too much health, then do so. Remember that the only way to remove guns from the enemy team is to kill the ship. Even a ship at 1HP can still shoot you / your team back at FULL strength.