We should compile some tips here.
Here's a few of mine:
Listen to your comm.
Buy welders and use it on your teammates to weld armor as Marine.
It is always better to do an objective (such as securing a res node) then to run around randomly killing things
Stick together, this will stop you from being ambushed, and this is especially important for marines.
Don't play comm until you have played around 20 or so games, you really need to understand the game inside out to be a good comm.