When writing an opening scene of a screenplay, your first decision is the subject. This should generally be something about which you know or about which nobody knows (pure fantasy or science fiction, for instance). If you have a strong interest in a subject, you probably know a fair amount about it already, though more research is always useful. If you are knowledgeable about and interested in one aspect of a story line, you can research the others to fill in the gaps in your knowledge. For instance, perhaps you are very interested in theater and have acted in and watched many plays. You decide to write about a murder which occurs during a theatrical performance. The other aspects of the plot, such as police procedure, can be the subject for your research, since you already have a feel for the setting.
The next decision should be whether you are writing a radio play or a screenplay. This will have a great impact on the type of stage directions you give and on the dialogue, since in the former case, you have to build up a picture in the listener's mind using sound alone. In both cases, you will need to think of an opening which grabs the attention of your audience. This might be a dramatic event, a memorable opening line, or a thoughtful reflection on the events that are about to unfold.