How do you file for discovery

There are various discovery mechanisms available to you.  For example, you can serve the plaintiff with interrogatories (written questions which plaintiff is required to answer under oath) but depending on what court you are in, these may be substantially limited.  You can serve a document request seeking production of relevant documents.  You can serve a demand for a bill of particulars (technically not discovery) providing more detail about the claim against you.  You can serve a notice of deposition where the plantiff would have to answer questions under oath.  You don't have to "file for discovery", but simply serve one or more of these discovery demands on the plaintiff.  It is only if there is some sort of dispute over discovery that the court would get involved.