Wear as much team apparel as possible. How is anyone supposed to know what team you like if you do not have their logo emblazoned on your chest at all times? When it comes to jerseys, choose someone who will be on the team for a very long time, you don't want some weird midseason five-team trade leaving you with the jersey of a guy who was just traded to your team's archrival. Don't know who your team's archrival is? Don't worry! Wearing all of your official team apparel will buy you some time to learn your team's history.
Bonus Tip: Look for team apparel at your local thrift store. Wearing an old shirt or jersey will make it look like you've been a fan for a while.

Wear hats. Hats are so important to jumping on the bandwagon that they deserve their own category. There are many different types and styles of hats, you've got the fitted hat, stretch fit hat, snap back hat, game hat, sideline hat, spring training hat, and throwback hat, just to name a few. You're going to want to buy them all. If you want to be on the bandwagon, hats are no longer just for bad hair days.
Bonus Tip: Wear your hat backwards so all of the losers left in the dust will be aware of the team you are now currently supporting.

Watch the games. (Sort of) The actual game can be so boring and dull, especially when the only reasons you have for liking the team are social. Don't feel any real pressure to watch the game as long as you are prepared to watch the game's highlights or read about it in the paper. If watching highlights and reading the paper are too much, just ask the first person you see in the morning what they thought of the game. Remember this answer and repeat it to anyone who accuses you of not watching the game.
Bonus Tip: If you ever find yourself in a situation where you are surrounded by real fans and have to watch the game, prove how much of a fan you are by yelling loudly at the coach, screaming loudly at officials, and cheering loudly at big plays. Basically, just do everything you hear the other fans do, but louder.

Say these things.
Before a game: "Big game tonight!" or "I'm feeling good about this one."
After a win: "Great game last night."
After a loss: "The officials really stole that one from us" or "Fire (coach's/manager's name)!" Miscellaneous: "(Archrival's team name) SUCKS!", "Our bullpen/special teams/third line really needs some help.", and "I read an article on that."
Bonus Tip: If someone calls you out for jumping on the bandwagon, tell them that your Dad has always been a fan of the team.

Now that you are on the bandwagon, don't forget to constantly update your Facebook status about things that are currently going on with your new favorite* team. If updating your own status is too much work, just like other people's statuses about the team.
Bonus Tip: Use a picture of the team's most loveable player as your Facebook picture. Better yet, Photoshop yourself into the picture.