The first thing you have to remember about screenwriting agents is they do not want to hear from you or read your scripts. They, more than any other entertainment agents or managers, have to deal with sizable inquiries that are overwhelming and the majority are not going to make it. The old assumption that everyone in Hollywood has a script is somewhat true, and you are not going to be any different to them.
Your first submission to them will never be your script because it will be sent back to you unopened in almost any case. This is because they do not want the legal recourse of what could happen with a script they have seen but choose not to represent. Unsolicited scripts could get them sued if a film is produced with some of the elements in your script, so they will not even open it to prove that they could not have looked at it. What you want to do is to call the assistant for the screenwriting agent and ask about the submission process. If you are lucky, or if you have some connection to them personally or professionally, you will be asked to send in a query letter. This screenplay query letter will give some information about your script, such as a treatment, and possibly information about you as a writer.
If they are interested then they will request a copy of the script, which is only the first step. At this point, you send a physical copy of the script, not an email or digital file, since this will protect you in terms of having parts of the work stolen. You may even want to have the work copyrighted or registered with the WGA ahead of time, but this is not something you need to make known to the agent.
The agent may not even read the script that has come in and hire a reader, which is a person hired to read and write coverage for scripts. This reader may just present the agent with a one or two sheet outlining what the script is about and whether or not they will believe the agent will be interested in it. If they are then interested, you may be signed by that agent for representation. This clears a major hurdle but is not a guarantee you will be hired to work as a professional writer in the film or television industry. Make sure you do not make contact with the agent unless you have completed work already that you think is of a superior quality and marketable nature.