Get a criminal defense attorney. If you cannot afford to hire a criminal defense attorney, apply for a public defender. Criminal defense is not a do-it-yourself project.
Since possession of marijuana carries a 6 month license suspension, the money you invest in the services of a lawyer will be well worth it if the marijuana charge is dropped (as it often is)
A drug offense can lead to a license suspension along with other serious implications. It is suggested that you immediately sit down with an attorney who can explain the process to you.
First, your question: The preliminary hearing before the Didtrict Judge is followed by a "formal arraignemnt" in the court of common pleas. In some counties, the next step is the trial list. In other counties there is a "pre-trial conference".
I agree with my colleagues, don't even think of representing yourself for the reasons stated. You need a lawyer to help you with this. You may want to discuss a possible Section 17 with whatever attorney you choose.