How can I sue if the landlord's leasing office opened my bill before I received the bill?

It may or may not be against the law because I don't know if you are talking about the landlord's bill to you or a bill from a third party. If the former, it was their bill. If the latter, were you still living there or had you moved? In either case, did the landlord open it inadvertently, not knowing it was too you because they just open envelopes. In those cases no harm, no foul. If they did it intentionally, then it may be a violation of federal criminal law; but unless their opening it caused you financial damage, I think the same rule applies - no harm, no foul.

It's a federal crime to open someone else mail. I would just remind them. It would not be worth your time and money to pay attorney's fees and court fees unless you were harmed financially.

First, what are your damages? You do have damages right? Liability and no damages is not much of a lawsuit, and even less if you are paying your own freight for counsel and costs. It's possible you are overreaching a bit.

Make a complaint with the postal service if it was mailed.