At the moment, there are a few platforms that allow you to target users by location, interests or demographic. When you set up an ad on Facebook, Instagram or other similar platforms, you will be asked for information that will narrow down the number of people who see it in their feed. You should be able to provide the information based on your knowledge of your target audience.
On Google AdWords, you can target users based on the keywords they type into Google. When creating an ad, first do some research into which keywords are the most popular.