I would start off with ONE SENTENCE that describes exactly what you need. You can add details later.

E.g., "I need an ambulance for a gunshot victim at 123 Main street in Anytown, Colorado"

"I need police assistance for a home invasion at (location)." Always give your complete location, including city/state (no need for zipcode though, they're not going to be responding via US mail).

That way, if you get cut off or start babbling or fall apart later, the dispatcher can play back the start of the recording (your calls are always recorded) and get things rolling. Hopefully you can get your head together enough when calling 911 to come up with one short, clear initial sentence that you're going to say.

Next up, I would give a callback phone number. They may already have this, they may not, but it's important so go ahead and state it specifically to cover the bases.

Later in the call you can give your name, the extent of any injuries, in a fire scenario if everybody has gotten out of the house, identifying/navigation info about your house - paint color, what type of cars are parked out front, etc.