How do you get a stage name?
Choose a name that distinctive. If you have a name that's duplicated multiple times in the phone book, you will likely run into issues with being confused with other actors of the same name, making your more likely to be seen as just another face in the crowd.
Choose a name that's memorable. Having a name that sticks out in someone's mind will help people remember, by name, a great performance that they saw you in. The last thing you want is for someone to say, "I saw this great actor playing the character Flagstaff in this performance and it was... shoot, I can't remember his name!"
Choose a name that is not to difficult to spell. Simply put, if your name is hard to write, people won't write it. Only slightly better will the instance that you name appears misspelled, which may cause confusion or focus on the name itself and not you or the performance.
Choose a name that is not difficult to pronounce. Similarly, a name that's hard to pronounce won't get talked about as much as a name that easily rolls the tongue with pleasure. Whatever name you choose, say it over and over to see if it's still easy to say.
Consider how your stage name will inspire you. Many actors find that their name not only helps them step into their character, but to step into the role of a "professional actor". To this end, choose a name that suggests you are a great actor, such as name inspired by your great role models.
Consider using your own name. If your name fits all of the above and works as an actor, why not keep it?