Let me start by saying that you should have a goal in mind if you are going to start paying for promotion. Additional thoughts:
– Keep in mind that you get almost no reach organically and Facebook is generally a “pay to play” platform.
– Small tests: Each time you post and pay to promote it, you get to see how it performs against an audience of your choosing. You probably have at least some “likes” so a few people will see it organically. If you boost it for even $1 a lot more will see it. If you just do this consistently with all your posts, you will start to see what works and doesn’t work. When you find one post that performs better (however you decide to define better), you can continue to boost it.
If it works for $1, spend $2, if it works for $2 spend $5 and so on… as long as it is meeting a goal.
Audience building: You can build audiences from people who click on your posts, watch your videos, click call to action buttons, or visit your site. If you boost and get a few clicks and boost again and get a few clicks, Facebook can start to understand the type of person who is your ideal audience. That’s really powerful and valuable. You can build an audience of only people who took an action, as well as a “lookalike” audience of people who Facebook *thinks* are similar to the people who took the action. These audiences have a tendency to be better than using interests and demographics.