Another device is using your IP address mac, what do I change my IP address too?
Often times these issues can be solved by rebooting ALL computers (iPhones, Macs, etc.) on your local LAN. A reboot causes DHCP to request a lease. And your router should only hand out ones that are unused. So every device should get a unique address

HOWEVER it is possible that you have manually assigned an IP address to one or more devices, so check for that. Make sure all are different and all are on DHCP.

ALSO possible, someone unwanted is on your network. Change the WiFi password just in case.

Failing all of that assign everything a static IP address like this

192.168.1.x

Where x is as follows

between 2 and 254
  unique (only one device has any one number)
  Your subnet mask is likely 255.255.255.0
  Your Gateway is likely 192.168.1.1 (your cable modem or WiFi access point)
  Your DNS will vary

The last three should be listed in your network System Preference and will be the same on all devices (only the IP address must be different). Select the connection (WiFi) and click Advanced, then the TCP/IP and DNS buttons. Record the subnet mask, gateway and DNS servers. Then fill that info in on all devices.

It would be a good idea, if you do this, to make a note of all the settings and which device gets which address. Keeping track can be a pain, which is why we have DHCP. It is supposed to hand out all that info automatically. So it is likely that there is a device on your network NOT set to DHCP.