Well, it depends on where you are in the world. In the US, the movie is already available in theaters. You can check the movie showtimes and purchase tickets at Atom Tickets:
https://www.atomtickets.com/movies/alien-covenant/213171
If you're outside of the US, then you can check with your local theater for showtimes.
If you're looking for home viewing options, Alien: Covenant is now available for rent or purchase on Digital HD from Amazon, iTunes, and Vudu. For DVD/Blu-ray, you can find a copy on Amazon, Best Buy, Target, and other major retailers.
