Once a lawsuit is filed, attorneys will want to continue their investigation into the facts. In a lawsuit, this process is called "discovery".
As part of the discovery process, the opposing attorney has the right to schedule your deposition. A deposition simply means that the opposing attorney gets to sit down with you in the same room and under oath, with a court reporter, and ask you questions regarding your claim.
Prepare? It is best to know the facts of the accident and the details of your injuries.
If there are medical records, are they accurate? Is there anything written in them that you will need to explain (Note: There almost always is..).
Depositions can last 4 hours in Arizona. If you have an attorney, he will sit down with you beforehand to explain things and prepare you.
You should be able to ask or tell your attorney anything. In depositions, the one main area that is off limits for the opposing lawyer are attorney/client communications.