are you playing on Normal?
Im assuming you are... just keep plugging at it, explore a bit and learn where the power ups are in your stage (the shield is pretty useful for wasting lots of baddies and capturing a control point, and all powerups respawn after a minute or so)... and yeah, theres not much esle to the game you just have to get quite good at it, learn the best way to attack certain control points, know when to keep trying to capture new spots or just hold out where you are until you win.... and dont forget to use your depth charges! they are really powerful and once you learn how to aim them well you can take out 3 baddies with one charge easy
The corsair is generally the best class to use, so level that one up to max first, the missiles shoot quick and give nice splash damamge also