First make sure your hub/switch is working. Give static ip's to two machines connected to the switch, make sure they are in the same subnet - then test connectivity between the two machines by using the ping command.
Once you have established that your switch is working fine focus your attention on the DHCP server. Make sure you have a connection through the switch between the clients and the server (using the static ip's) Then set the clients to obtain an ip address automatically. Leave the server with a static ip address. secondly create the scope on the server making sure that the scope is apart of the same subnet as the DHCP server, activate the scope and authorise the server you should see green arrows right the way through. Try renew the ip address on the clients and see what happens.
keep me posted