The best thing i would do is try avoiding every thing he tries hitting you with. Hit him with every item you can get your hands on, smashes are also great. Air attacks will work too but when he raises up get ready to block.
..anyone have any tips for me? I'm about to start fighting Tabuu in The Subspace Emissary. My best characters are Pit, Zelda, Ice Climbers, and R.O.B.
Tabuu... Unless you're playing on Very Hard or Intense, you should be able to handle 'em after a few tries.
Character choice isn't too important, so long as you stay away from the slow and/or heavy fighters. Tabuu's attacks are all more than powerful enough to kill off a character regardless of weight, unless you're playing on Easy or lower.
The most important part of fighting Tabuu is dodging his "off-waves" with sidestep dodges. If you don't know what a sidestep dodge is, it's when you pull up your shield and push down on your control stick. It gives you "invincibility frames", which is to say you dodge any moves that come your way while you have said frames.
When Tabuu teleports to the center of the screen, you can't hit him, and he's charging up energy, this is when you know you need to dodge the off-waves. Once he throws out his wings three of these waves will go across the entire screen in succession; their speed is faster the higher the difficulty you're playing on. The slower the off-waves go, the better you need to time each sidestep. If you're too early, you'll get hit. If you're too late, the off-wave goes right through your shield and sends you flying.
On Hard mode and higher, the off-waves are generally a one-shot for most characters, and on Intense mode it's a guaranteed kill. He does the off-waves every few normal attacks too, so if you can't master dodging the attack, you will need to rush to kill him.
The off-waves aside, just be very wary of Tabuu's whereabouts. You won't be able to dodge too many of his attacks until you get some practice in since you won't know what attack he's about to bring out. You can maximize your lucky dodges by being ready to jump into the air as high as you can/air dodge at all times, though. Be vigilante and careful, basically.