Where to start, where to start....
First, get a real legal opinion. Even if it's just a free opinion from a university or clinic type thing, it will help, as any cabable attorney will be able to ask a multitude of clarifying questions, and then give you legitimate advice.
Second, and this is very important, you need to find people that saw what happened. It will be significantly helpful to you if you do not know them in any way, save that they witnessed either (A) the guy getting thrown out, (B) the guy provoking others, or (C), this guy pushing you. His friends may or may not testify for him. If they do, and they lie, chances are high they will get caught lying. Besides, they are his friends. Your witnesses will be people you've never even seen before. Likely the best people for this will be the employees that were working that night, as they will not have been drinking (hopefully), and will be construed by the judge to be completely unbiased.
Third, obtain the police report immediately. It would not be a bad idea to file charges against this guy, but you need to see what the cop reported. He may not have seen you get pushed, but the report may say "I observed 4 people exit the bar. While they were walking, they appeared to have some type of conversation with a fifth individual. One of the five turned around, and the fifth individual struck him." This would significantly hurt your case.
Most importantly, consult an attorney.
This whole story actually reminds me of my night last night. I was out with some friends, when some asshole at a pizza stand called my friend a jackass. I turned around and shoved him so he would know he can't get away with that shit, and then the little vagina punched me in the mouth before running away like a little girl. I was gonna chase him, since he hits like a girl too, and it didn't hurt, but then the cops showed up so I had to bail. I'm gonna find that kid, though, and get him good.