1. First of all, you have to assign an IP address on the VLAN interface of the switch (if it is a Layer 3 switch).
switch(config)#interface vlan 1
switch(config-if)#ip address 10.20.30.1 255.255.255.0
2. Then you have to create a DHCP IP address pool for the IP addresses you want to assign to your clients.
switch(config)# ip dhcp pool mypool
3. You have to specify the network and subnet for the addresses you want to use from the pool.
switch(dhcp-config)# network 10.20.30.0
4. Then you can specify the DNS, the gateway, and other options to send to the clients.