At the moment, Spin, Bird, Lime, and Lyft are the major scooter services in Atlanta. However, that could change, as the Atlanta city government seeks to establish a regulatory scheme for the dockless scooters, and it is not clear who will emerge from the process with the necessary permissions to operate in the city.
As for the how, all of the dockless scooter operators have an app. You go to their website, download the app, agree to the legal conditions listed in the app, and away you go. You register a payment method with the company through the app. When you’re ready to rent a scooter, assuming you don’t stumble across one coming out of the MARTA station, the app will tell you where the nearest scooter is located.