It says someone is using my IP address on my MacBook Pro

This message is telling you that the IP address, that would be assigned to your MacBook, is currently in use by another device on the local network that you are trying to access with this notebook.

Is the network you are trying to connect it to, you home network, or somewhere else?

Regardless, let's take a quick look at your notebook's network settings. To do so, go to System Preferences > Network > Wi-Fi. What is the setting for Configure IPv4? The default would be: Using DHCP

What Tesserax said.

To give more tailored advice, please tell us: is this a home mac?

Open System Preferences->Network, select Wifi or Ethernet (whatever is on top) in the left side, and post a screenshot, so we can see how your network is set up. If this is a home mac, it should set as DHCP, like Tesserax said. In that case, the DHCP server (usually your router) distributes IP addresses automatically your mac and other devices (like phones, or ipads, or even smart tv) in your home network.