What you mainly need is to create a wallet, we suggest you to create it in metamask.io a wallet that allows you to be installed in your preferred browser (Google Chrome) and is very easy to install. Once you have installed it you will have to start the registration process which is also extremely easy.  https://metamask.io/ When you have registered and have your account created you must among other things save your secure password and once you have it you can enter opensea.io, there automatically when you enter the profile you must select metamask and create your account without any problem.  Remember that to acquire your NFT you must have sufficient funds in your wallet.