My advice is to first establish your desires. Are you seeking a lifestyle or a Professional Mistress? Would you prefer session-based, or 24/7 D/s? What are your long-term expectations from this relationship? Once you understand your needs better, you can then set about searching for the Mistress of your dreams. If you are confused about what you want, this will be perceived by any potential Mistress and in all likelihood, your applications won't be successful.
There are many popular BDSM community sites online, where thousands of Dominant women and Mistresses are often grouped by location to make finding a local Domme even easier. There are also Facebook FemDom groups as well as various hashtags on Twitter, which enable you to quickly find people with the same interests, as well as connecting you with many Dominant women and Mistresses.
Once a Mistress catches your eye, pay particular attention to your initial contact. Chances are, she receives many messages from hopeful submissives. The reality of the situation is that it is ultimately her choice whether she accepts or rejects you. Read her profile to follow any specific instructions, especially when it comes to how she would like to be addressed. Refrain from making demands or attempting to brag or manipulate her in any way.
Above all, remember your initial desire to serve a Mistress. It's her needs that you should now be thinking about, not your own fantasies. Find out how your potential Mistress likes to be worshiped and treated and personalize your service plea.