You should think of a Realtor the moment you think of selling your home - even if you're not 100% sure yet. The real estate agent will fill you in with as much information as you want about the market condition, your home's estimated value, staging ideas and the list goes on.
You will not know every requirement for selling a home, most especially if this is your first time selling. You may, however, start listing some things that you may want to think about when choosing your Realtor. Some of them are listed in this free report.