I think you're about 5 years too late to make any money that way! Just about every general purpose sort of domain has already been snapped up, and the courts nowadays take a dim view of domains which they feel have clearly been registered purely with a view to making money from what might be considered a more "legitimate" user.
When you buy a domain, you have to renew it regularly, and the term varies according to the type (e.g. com, co.uk, etc.) but, if you don't intend to actually use it, there are no other costs involved. However, IF you don't actually use it, you are leaving yourself wide open to the problems detailed above.
If you google Buy Domain or Register Domain, you'll find loads of links.