A scenario is a sketch or outline of a story; it may be more or less detailed, but it does not specify everything that happens, just enough for a reader to have an idea of what is intended. A scenario is typically used to present an idea for a production for which the author is seeking funding, but in the last couple of generations the term has been extended to descriptions of imaginary problems for which readers are invited to find solutions.
A script is a more or less full specification of dialogue and action for a stage play or movie (when the script is intended for a movie it is usually called a screenplay) or other medium; in my shop, for instance, we use script for multimedia presentations and websites as well as videos. Depending on circumstances there will be versions for different purposes which have different sorts of detail. In the theatre the director and actors will go into rehearsal with a script in which the dialogue is specified, but action only sketched; as rehearsals go forward and the action is worked out, more detail is added. Set, lighting and sound cues are added separately; all this goes into the prompt script employed by the stage manager during the run. If the play is subsequently published for general readers, the author may add descriptive details which enable the reader to envision the action more clearly.
In your case, screenplay would be inappropriate; whether you are composing a scenario or a script will depend on what purpose the work is to serve.