Connect from the server browser. If the server is full you may be put in the queue as a spectator; you join automatically when a slot opens. See Ready check and queue.
In the ready room, walk into a door (Marines / Aliens / Random) or use F5 / F6. Teams are usually shuffled for balance before the round, so your pick may be changed. See Teams and shuffle.
When a round is about to start, click READY in the ready check.
You are a Skulk. Don't charge marines in the open: go around, approach from walls and ceilings, bite from behind.
Parasite (2) marines you see. Your whole team then sees them.
Kill undefended extractors and power nodes: that hurts the marines more than a kill.
Don't go Gorge at the start unless your team asks. Too many gorges is a classic way to lose. (The alien team is told who goes gorge in the first minute.)
Save your res. Your first Fade or Lerk should be bought when you can afford to lose it.