I want to buy an Xbox One, do I need a membership to play games like Fortnite or other games that can be bought in the store?
It has nothing to do with the store.
However, if you want to play online with others, you need Xbox live gold.
If you want to play games with your Xbox online via an Internet connection, an Xbox Live Gold subscription is a basic requirement and mandatory. From there, the service offers a number of other worthwhile advantages:
4 free games per month as part of the Games with Gold program.
Access to the multiplayer community with features such as matchmaking, which you can use e.g. Mediated with players of the same skill.
Discounts of up to 75% in the Xbox Store etc. At sales.
Comfort functions such as Xbox Live Parties, in which you can chat with friends across games using voice chat.
Sometimes earlier access to demos or beta versions.
