Starting point would be selecting an Avr, Yamaha / Onkyo / Denon to name few. An Avr that supports 7.1 or 7.2 speakers and Atmos. Atmos can be tricky to get the whole experience with additional speakers beyond 7.2 but movies do default to audio tracks suitable to what the Avr can support and speakers connected to it.
Typical 7.1 or .2 setup consists of 2 fronts, centre, rear sides (back corners of the room), rear backs (beside your soafer) and subwoofers. One subwoofer placed near the front and the other behind to complete the bass, one isn't enough imo. 5.1 systems are fine too, minus the back soafer speakers. Most subwoofers can be daisy chained having rca outputs to do that so an Avr doesn't necessarily have to have .2 outputs to run multiple subwoofers. My Avr doesn't (7.1) and i got two subs daisy chained. Avr's that support .2 just means a movie can utilise two subs differently instead of it being linear, still sounds good though.
Speakers/subwoofers are personal preferences and would be best for you to visit a store that deals in home entertainment equipment so they can demo and explain things like speaker position and calibration - calibration done with a mic placed where you sit. Most Avr's come with it and easy to do and sounds great afterwards because sound is central not able to pick out any individual speaker played simultaneously, ie music. Can buy stuff online sure, tower speakers for fronts, bookshelf speakers for rears, tower speakers for rear backs, centre speaker and a subwoofer but you won't know how they sound before buying, only relying on other experiences.
Other thoughts.
Speaker stands to place rear speakers on, or wall shelf.
Avr that is heavy is a good indicator of quality.