setting up a trust relationship

I need to set up a trust relationship between our Germany office and NewJersey office. The offices are connected with a VPN tunnel and each office has their own separate domain each in it’s own forest. Do the domains need to be in the same forest in order for a trust relationship to be established? And if so, how do i merge the domains into the same forest? I basically need to set it up so that the New Jersey domain trusts the Germany domain. What steps need to be taken to actually set up the trust relationship?