Facebook advertising is a big deal nowadays.
I'm not talking about boosting your posts (don't do that). I'm talking about building a true Facebook ad -- one where you define your target market, choose your image assets, and write your full message.
Everyone is trying to sell or convince people on Facebook. But when it comes to building an ad, well, some think of it as a dark art.
So lets break it down.
Fundamentally, I'm going to guess that what you are trying to do is persuade your customer to "do something" like buy/subscribe/join/etc. With that in mind, I would like to recommend one of the most influential books I've ever read on this topic: Seducing Strangers by Josh Weltman.
Seducing Strangers breaks down the principles of persuasion in the context of marketing and selling products or services. By the end of the book you will be looking at your ad/message from your customers perspective, helping your customers understand the value your product provides instead of the features it has.
A relevant lesson from the book on social media advertising is this: The Internet is a wide-open platform on which users are trying to navigate to a better version of themselves. Every click they make is to learn something or connect with someone in a meaningful way.
Understand this principle and the 50 or so other principles in this book before trying some of the techniques in other answers. Without the fundamental principles at play in your message, your ad will just be experimentation (and a waste of money).
Full Book Review Here: You Don’t Know How to Sell Until You’ve Read This Book
Principles aside, it's helpful to understand that when people are on Facebook, they are usually there to see what their friends are up to, scrolling through their feed, looking for funny images or interesting updates.
Thus, creating a Facebook ad that closely mirror a typical "friend post" gets a lot of interaction.
What does that mean?
It means using a photo of a person so they can relate - a selfie or something similar - like one would expect to see on their wall. It means using a "real photo", not a stock picture photoshopped to perfection.
Once you have their attention with a wall-worthy picture, you need a captivating headline. Questions are good because they engage. So are powerful statements. Remember, the viewer is looking to click on things that will entertain, enlighten, or take them one step closer to their ideal self.
Make sure your headline promises one of these to them. Most importantly, make sure you follow through on that promise.
Finally, some subtext is appropriate in a Facebook ad. This is the descriptive text under your headline. It should be friendly, and read much like a typical Facebook users post. Anything too formal won't fit the platform.
Combine these elements and throw $5 or $10 over two days to see how the ad performs. If your ad is getting good click through rates, great, run it again! If not, make a small tweak and run it again for two more days.
Rinse and repeat.
One final thought: Read How Do I Advertise My Business? and How Much Should I Budget For An Effective Facebook Ad? for a complete view of the Facebook advertising scene, and how to start off on the right foot.