You can create an ad in Ads Manager, or by boosting a post. Boosted (or promoted) posts are ads you create from posts on your Facebook Page. Boosted posts show up as ads in your audience's Feed. They're a good way to reach more people who are on Facebook but don't currently follow your Page. Boosted posts can help you raise awareness about your cause, and motivate people to visit your Page and follow you, which in turn will help grow your community.
When you boost a post, you can choose the audience you want to see it, where you want to place it, the duration of your post, and how much you want to spend on it. If you want more options to customise an ad, it's a good idea to create your ad through Ads Manager.