First you need to get your Vita online. Fire it up and create a PSN ID or attach the one you use for PS3 if you have one. Once you've done that you will need to get onto your wireless network. Go into settings - wifi - and enter your network's password. Now that you're online you will need games that have multiplayer to play against other people.
As for posting, I'm not sure what you mean. Where do you want to post?