100% the first thing you want to focus on. . . CLEANLINESS. No, we don’t advise going to a backyard/garage/bedroom at your mates friends place to get your ink done. . . You are just asking for a poor tattoo with a side of infection.
Most states require a license and a thorough knowledge of Infection Prevention to be able to trade as a tattoo artist/business. Take some time to check out the rules about being a tattoo artist in your state - Google will help you with that one.
Next is what you want done and the style you are interested in, there is no use going to an artist who only specializes in Neo-Traditional artwork when you want a Black and Grey portrait of your Grandmother. Check out their social media, Facebook, Instagram, Pintrest if you have too! Most artists will list what that specialize in.
If they look like the right fit for what you want to get done, see if they will do a consultation on your idea. Great tattoo artists will give you their time and help you decide if they are the right person to do the artwork that you want. There are always reviews, but don’t always judge on what others spew on social media (unless it’s everyone, then maybe not the right choice).