TMs are earned through usage in commerce, you then register the eraned TM and have it registered.
Are you currently using the domains to a point where they gained secondary meaning in the marketplace? Would the TM be on the name within the domain or on the whole domain itself?
Thats the site for the US. You can actually fill out forms online yourself and just pay the normal filing fee. I think it's like $325.
You could probably use some type of service like LegalZoom.com, but I don't know how much easier that would be than using the government site itself. Plus they charge you a pretty decent fee.
If you have no current use, you would probably file for a ITU (Intent to use) TM, then it doesn't become active until you show actual usage. And you have a limited amount of time to do it (i don't remember the amount of time).