CNFT.IO is the mainly used one, creating custom NFT contracts have not been widely introduced in Cardano Blockchain, however to create NFT collection you can make Policy ID stable and create a collection.
To add a collection and get verified on CNFT.io you will need to add ur policy id to their github and request a pull

https://github.com/Cardano-NFTs/policyIDs

It is pretty easy to create NFTs on Cardano but the secondary payments is still not solved exactly yet, some other trusted marketplaces are:
EXNFT io
Tokhun io
CNFT io
NFTmaker io
Nextcnft com (this is not launched as far as I remember)

Good luck and hope u create a nice collection.