When my friend got her tongue pierced she was actually told to avoid dairy. But, I dunno... I can DEFINITELY say that you should avoid soda, orange juice/citrus and other highly acidic drinks.
Just follow whatever the piercer's instructions were very closely. I'd also recommend making sure you brush your teeth gently twice a day and floss at least once a day (you'd be surprised at how much bacteria can build up between your teeth.) Water is the most non-irritating liquid you can drink. A saltwater rinse is also effective. I imagine mouthwash might be overly drying, but I'm not a piercer, so...
Oh, and never buy cheap jewellery made out of nickel and the like. That shit always makes my ears flare up and sometimes bleed. It's just my ears, so I don't necessarily mind wearing cheap jewellery there, but your tongue is important. Even if you don't initially react to cheap metal you could develop an allergy for it.
http://www.essortment.com/all/tonguepiercing_rjsl.htm Basically just do all the same stuff you do when you have your wisdom teeth removed.
Also, if you're willing to wait it's better to get your tattoo in the winter when it's dry. Not that you can't get one now, but it makes caring for it slightly easier.