This walkthrough assumes that you are using OS X Server 10.5 on the Advanced Configuration.
To start off, you need to open Workgroup Manager (/Applications/Server/Workgroup Manager) and it will ask you to enter your server password. Enter it, and at the top, click on the plus sign to add a new user. Set the name and password, and then click the privileges tab and set privileges to full. That is all you need to do to set up the user. Click save.
At the top of the workgroup manager window is an icon that says Server Admin. Click that, and it takes you to the server admin program. Select your server, enter the password, and you will be on the General Overview about services and specs about your server. Select file sharing, and then slect a drive to share. Then select the settings icon on top. Under the services tab, select AFP for sharing with Mac's and add SMB if you want to share with window's as well. Select Access, and add the user that you created in step 1 to the list at the right for the AFP service (and SMB if you so desire.) Click save, and now you can access your server from your other machines