This is a great Google tool and third-party tool that provides you with a list of all the possible topics that people are typing into Google. Start by brainstorming for 5 or 10 minutes – what are the top 10, 20, or 30 keywords you think your target audience are searching for? Then put them into the keyword planner, and that keyword planner will spit out 4000 possible ideas for you to consider.
Then you want to start sifting through them. This means deleting, rearranging, sorting, and making sure that you end up with a good list of 200 – 400 keywords that are relevant to your business.