Before you can even start earning some money and making profits with selling domains you will have to find a domain registrar. There are several out there in the world wide web but not every one of them is as good as they're stating on their different internet sites. So choose wisely. The domain shouldn't cost more than $ 10.00 max per year. When I register a domain I always go for the com's. Don't go for the org's, net's etc., stay with the com's because they're the most used domain endings in the web and people are willing to pay or invest more money for the com's than for any other domain endings.
The most valuable domain names you can register are two letter domains. Whenever you will have the opportunity to register a two letter domain, go for it, it doesn't matter which letters the two letters are, go and register it and it will bring you thousands and thousands of dollars. But make sure it's a com domain. Same goes with three letter domains, these kinds of domain names are very rare too and people are willing to pay thousands of dollars. You can go to any domain registrar and try to register one of these domains. But great luck has to be with you to get a hand one on because most aren't available anymore. Sometimes when I find a bit of time I go and do some testing on three letter domains. One time I was lucky and ran into an available three letter domain. I sold it later on for $4,300.-
Most of the "One Word" domains aren't available anymore also. Many people call these kinds of domains dictionary domains. So, you could go ahead, grab any dictionary, open a page and try to register any word of the dictionary. For example, try to register furniture.com or kitchen.com or umbrella.com. It really doesn't matter; you will see that it's very hard to register one of the dictionary domains. I think you got it! Whenever you stumble across an available dictionary name go ahead and register it. This domain will definitely make you good money.
For now it's even hard to register "Two Word" domains - logical two word domains. You can try to register for example kitchendoor or schoolbus or truckdriver. Whenever you can get a hand on one of these domains register it and make an extra income.