Avoid getting grabbed and avoid getting grabbed. Oh, and keep it aerial. Wario is a an aerial character after all. His range is bad so you don't want to get zoned. Try to get in your opponents and cause mischief. His stache isn't only for show y'know...
Nah but it's hard to give concrete tips. If you feel Wario is a character for you then go ahead. Otherwise don't put effort in him. He's an odd character. You must have patient when playing him and be careful of what you are doing, which attack to use, where to land, taking risks, trying to punish etc. The sole reason I main Wario is because he is the most fun character to play with.
Sometimes his issues are disappointing but on the other hand his strengths makes up for his weaknesses but not entirely. What I mean is that while Wario can be hard to grab (if you play him slippery) thanks to his aerial mobility, when he gets grabbed you're for the most part screwed. His range is also bad but is somewhat complementing with Wario having good air speed. So Wario is a really balanced character.
What else can I say - practice with him. Oh, and do not really get grabbed cause you will just suffer so hard from it.