The advice you have received thusfar has been OK, but the easiest thing to do would be to watch Mike Smart's global summit presentation, which takes you from literally nothing, to having a (nearly) fully fleshed out REST API in a simple case. See that here:
If this is your first time working with REST Applications, this is the best place to start. Then, if you'd like, you can continue with an online learning course, where Mike is going to expand on this to include some best practices. Announcement for that is here:
Hopefully we'll see you there!