Having the right domain name is amazingly important.
In fact, it's so important that many businesses choose their business name based on the domain name available.
The absolute cheapest and easiest way to register a domain is using NameCheap.
Simply navigate to their website (link above for your convenience) then start searching for some names.
If your idea is taken already (chances are it is) then start searching for some alternatives.
For example, if your idea CoolRealEstate.com is taken, try: SellCoolRealEstate.com or RealCoolRealEstate.com.
It can be really frustrating to keep searching names and have them all taken. Don't worry though - You will find something, trust me.