On the Mac to be shared, go to System Preferences
Sharing pane. Check the box next to +File Sharing+ to turn it ON.
On the other Mac, if it has Leopard or Snow Leopard, open any Finder window. Look under
SHARED in the sidebar. The Mac being shared should appear there. Select it. Over to the right, click the button that says +Connect As+. Enter the user name and password for the user on the shared Mac and connect.
You can do this in either direction. If the "laptop" is Windows, you can do file sharing with it too, but it takes more steps to set it up.