First of all, you have to decide whether you really want to NFT or sell it yourself. If you want to sell yourself, the steps are a bit more complicated and longer. After that, you have to choose the platform and payment method.
If you are going to make it yourself and sell it yourself, you need to connect your wallet to a platform such as OpenSea where NFT can be sold. You can then add your own collection from the Create button in OpenSea.
Of course, for this, a certain design or design series must be determined beforehand. It is also very important to choose the NFT you want to sell.
And as for the other situation. If you just want NFT to add to market by designing and selling to others. This time, you can work for them by contracting with companies. So you don't have to deal with gas-fees when selling on Ethereum.