To create an NFT on OpenSea, first start by creating a collection.
If you don't have a collection, start here.
From the collection homepage, select Add New Item.
Upload your file (see maximum file size) and complete the NFT details.
The supply default is 1, but you can create fungible items (with many copies) by following the steps here.
Your item is now created, select Visit to be redirected to the collection.
Your NFT item(s) can be found in your collections and from your profile page under the tab Wallet.