My advice is to first shop for a bike shop.

Audition the bike shops in your area until you find the one with the staff person that you can identify with and who asks the "right" questions. There are new styles of bikes produced every year to fit an ever narrower segments of users. Find the "right" sales consultant, buy a brand that they carry, and you'll never go wrong.

Buying a used bike when you're inexperienced is hard. In addition to all the different styles of bicycles, bikes also come in sizes. Besides having the knowledge to evaluate a used bicycle, you also need to know when to walk away from a bike that's the wrong size.