I've pressed charges against this guy for stealing, now I'm going to court and he is being charged with petty theft, what questions will they ask me. And do they do background checks on the victim who pressed the charges.

They will ask you to tell what happened and why you believe the defendant committed the crime. The prosecutor should be going over your testimony with you.

The prosecutor may or may not do a background check, but there is a good chance the defense will.

You can expect the defense attorney to have found any and every bad thing you have ever done, and he will try to use it, to discredit you.

The DA should have discussed what they are gong to ask. Assuming it was your property that was stolen, they will have you tell what happened, perhaps prove the material was yours and so on.