@socphoenix@On RDP server and client is Microsoft built-in, but bound to license restrictions. In general, 2 concurrent rdp sessions on one machine are possible without a need to extend the standard Windows license / price. You can add and license the rdp services role / terminalserver role to a machine setup and have more flexibility: more users, more administration of user sessions.
If your target is a Windows machine, you need a user that is a member of the target machine's local rdp or local admin security group, either directly or via a domain security group, start a client software (below) depending on your machine's operating system, enter the target machine's address (ip or name) and user name and password, and have a desktop session on the target machine in a window to remote comtrol it. You might need a VPN or citrix connection to the target's domain and you might need to qualify a user domain of your login credentials, but that's it in general.