No one can help me about this subject?
1) To add other payment method you need a plugin, no idea how to do it.
2) Go to the Configuration > Filebase > Selling and enable selling of files.
Then you can use search bar and search for Can sell files and edit the permissions according to your needs.
3) What do you mean by paid content? Articles? Forums? You could use the Paid Subscriptions and set permissions for your subscribers user groups. For selling files you can use the filebase, but you could also use the paid subscriptions. It's up to you.