The first step is to go to the Support Forums. That's where it lists all the categories you have to choose from.
Then, you can scroll through those and see which category fits your post topic best. For instance, if you want to want to make a post about a relationship problem, you'd click on "Relationships & Dating."
Then in the lefthand corner, there should be a button saying "New Thread." Click that, type everything out, and post, and you should be all set!
If my explanation was bad, please do tell me!
If it is, I can always take some screenshots to help you.