I am developing a tool but It's really difficult to get a name. I'm wondering if I can use a domain like xptoapp.com without infringement the trademark "xpto" if it exists. How does this work? I see so many xptoapp.com where they don't have the trademark of xpto.
You should just ask the company. In most cases they will tell you that the intent is to not use their name to suggest that the product was created by them, but that it is fine to use their name in the name of your product for description purposes.
Check out twitter's usage policy (scroll to the domain part). They actually don't want you to use their name in the name of your product but they allow you to use Tw- and Twit-