When choosing your tags for a post you want to make sure they have something to do with what you wrote about. When Im kinda lost on tags, Ill open a second screen to keep things simple and go to the list of tags, Ill scan through them and choose ones that make sense with my post. Its important to make sure that they make sense with your post because if someone is looking through the tag of their choice, they are looking for those specific posts and if yours is correct than they will definitely check yours out. Many people use the tag list as a search tool to find their specific interest so you dont want to misplace your post in a tag that doesnt work.