If you read every topic and reply to anything that interests you, you will probably get a lot just from that.
Other ideas:
- Start new topics about anything that's on your mind
- Ask questions about anything you don't understand
- Answer questions in deck help
- Make posts about decks you've made or decks you want help with (just make sure to search forums first you don't want to duplicate someone else's deck)
But the best way is probably to get involved in an active forum topic (i recommend the philosophy/religion sub-forums. Its quite easy to get drawn into a debate and have your post count shoot up by 50 or more literally overnight).
Anything except spamming the forums is fine. To reiterate, just log into the forums whenever you have a free moment and say what's on your mind