There are no hard and fast rules for this, but here goes:
Obviously you'll need /boot /swap and /. The size of these will depend on your hardware and what you intend to install.
/var: If you're planning on running services like FTP or http that use /var, then it's a good idea to put this on a seperate partition. Again, size depends on how much data you're planning to serve up.
/home: A lot of people recommend putting /home on a seperate partition and this is particularly true if your server is going to have lots of user accounts.
/tmp: Always a good idea so /tmp can't fill your system and cause a crash.
/etc: If you want to lock down your server once installed, you can place /etc on a seperate partition and mount it as read-only.
That should get you started