Help minting NFTs please

I have stupid questions- I see that opensea and mintable allow you to mint for free, but opnesea I have to pay $100 or so to sell it on the marketplace. Do I just have to have that amount in my wallet or do I have to spend the gas to list it? How do I make sure there is royalty fees set to the NFT? I don't really understand how someone is making money by minting a series of 50 or so NFTs if it cost 100-200 USD each one and for a newcomer there is a probability it will never sell, which means you could just toss away thousands of dollars. Any help is greatly appreciated