First you will need a crate. We prefer wire crates that the dog can see out of and have good ventilation.
You can choose from two door or one door, depending on how you want to arrange the crate.
To buy the correct size crate for your puppy, you want to estimate the size they’ll be as an adult dog and buy one large enough for that, along with a divider to make it smaller until your puppy grows.
You could buy a smaller crate and upgrade later on, but I think that’s just unnecessary spending.