When you write an article you should always target only one keyword! The keyowrd can be a sentence that you type into google and the sentece where you want to show up in google will be your keyword.
How do I choose a keyword?
There are many keyword tools that help you with this problem. You can use the free Google KeywordPlanner or for example what I use... Jaaxy. It's not for free, it does cost $19/month but it is really easy too handle and has some nice functions!
Anyways, you want to look for a keyword that hasn't that much competition so it's easier to rank for but at the same time the keyword with the most traffic. In Jaaxy there is an own SEO score where traffic and competition are put together in one number.
What do I do with the Keyword?
Go to the next page, there I will tell you what to do with the keyword!