Firstly, you should finish your draft, and I mean finished so that you want people to read it.
Then...start with a quiet, comfortable place to hold your reading. Like this one, maybe.
You'll want to print your script a day or two beforehand, and highlight your readers' parts. (Highlighting takes time, so factor that in!)
It usually makes sense to, when possible, match characters with your readers' personalities. As in, don't give the gregarious upstart role to the shy wallflower.
Don't be afraid to have people reading multiple roles.
But...YOU might not want to read at all, so that you can take notes.
It is helpful to have your readers talk about what they liked or didn't like, or what they didn't understand. There is a fine art to not taking anything too personally here--remember, they are trying to help you!
Lastly, have fun! The point isn't to stress out over the details. Listen to your work, learn from it, and appreciate the time that others are contributing to your success.