How do I change my IP address? Because someone is using mine.

While at the above, you might want to change Configure IPv4 to Using DHCP.

You should only get an IP conflict if you've got static IP addresses assigned on your home network or if you've got two DHCP servers handing out overlapping address pools.

With that said, to check your IP Address if it's static:

1. Launch System Preferences from the Apple menu
2. Click on “Network”
3. Click on the protocol you are using, let’s say we’re using Wi-Fi with a wireless connection so click on “Wi-Fi” so that the network interface is selected, and then click on the “Advanced” button in the lower right corner
4. Click on the “TCP/IP” tab
5. From here, you should be able to view your static IP address.

You can also go into the terminal and do an ifconfig /release and ifconfig /renew to do a release/renew of your IP