I work in a business that goes to small claims court (or here in GA, magistrate court) very frequently. IANAL.
The first thing you need to do is find out how the court system works in your area. You shouldn’t need representation but you do need documentation, of course. In your case, it seems like you would need a copy of your lease and the evidence that she has been overcharging you.
What happens here is that we file what is called a “statement of claim”. The claim states the plaintiff and defendant, addresses for both parties, damages sought (plus court costs and attorney’s fees, when allowed). The court may require you to submit supporting documentation at the same time. The court in my county does not, but others in our state do and it’s very important and time-saving to find out all of their stipulations beforehand. Most courts are even going online now. In many counties in GA, you can even e-file a SOC (statement of claim). The fees must be paid by you up front, but you can ask the judge (in my experience) to have the defendant pay those back to you if you are awarded judgment. In GA, the fees in magistrate court range from about $50 to $130.50 in Fulton County. That usually includes service fees. (In other states, it can be significantly different, it’s only $38 in some Alabama courts.)
The defendant will have to be served by a process server or the sheriff’s department, depending on how your county handles that. Once she is served, there will be a time period where she is allowed to respond (in GA that’s 30 days plus a 15 day grace period. In AL it’s 15 days). If she fails to respond, a hearing may be set up (depending on the county court’s rules) or you may be automatically issued a default judgment.
If you get a judgment, and she doesn’t want to pay you could have a hard time collecting. Depending on the size of the claim, you may consider hiring a collection agency to collect the judgment for you, assuming she is gainfully employed. They would have to file a garnishment against her wages (which includes more court fees…typically in GA around $65 and in AL almost always $30). I do believe you would be able to file a garnishment yourself, but it can be a complicated procedure in some courts and generally requires working knowledge of their terms, procedures, and forms. Provided she makes enough money to garnish and is not paying another garnishment or the IRS or child support, 1/4 of her take home pay would be garnished and would come to you, less the collection agency’s cut, if you employed one. It takes about 30 days for the court to forward the money they receive from the employer after the first answer (a garnishment term meaning answer of defendant’s employer to the court of an amount they will send in compliance with the garnishment order), which in GA is required to be less than 45 days. There are other more complicated scenarios whereby if she is employed and her employers do not answer the garnishment, they could become liable for the debt themselves but it’s best not to get too far ahead of yourself. Also, I didn’t look to see what state you are in. FL is very difficult to garnish in and TX does not allow this time of garnishment, so that may be a factor as well.
You are potentially looking at several months to get your money back. Also, you didn’t mention when this all started and different states have different rules about the length of time you have to pursue different types of suits. You may want to check the statute of limitations in your area.
(IAstillNAL, so please check the laws and guidelines in your jurisdiction carefully. Call the magistrate/small claims clerk’s office and ask lots of questions. They will tell you how to do things in their area but cannot and will not give legal advice.)