It all depends on how you want to do it. You can use ftp or ssh or telnet or tightvnc. On the same network, and especially if using wireless, I prefer ssh as it is much more secure than the others.
 
First, you'll need to make sure both boxes have openssh server and client software on them. Check your firewall and make sure it allows ssh to go through as "trusted." Once all this is done, open up a terminal and find out the ip of both boxes if you don't already know. As root do:
 
/sbin/ifconfig [enter]
 
On each box respectively. This will give you their ip addresses.
 
Let's pretend the user name on the server box you want to contact is bob. Let's also pretend the ip of that server box is 192.168.1.101. Go:
 
ssh bob@192.168.1.101 [enter]
 
The server will then ask for bob's password. Give it and press enter. That's it, you're in!