Question about backing up my MacBookPro

So I went out and bought an external harddrive, or I'm assuming that's what it is. It says it is "Add-on storage for your PC" it's from Seagate Expansion, and it's 750GB. Plugged it in to my computer, and something popped up asking if I wanted to use it to back up my files with timemachine, I said yes.

But now it says:
Are you sure you want to erase the backup disk “Seagate Expansion Drive”? Erasing will destroy all information on the disk and can’t be undone.The disk must be erased before it can be used for Time Machine backups because it has an incompatible file system.

Do I erase or not? I just spent 100$ on the disk so I want to make sure erasing whatever's on it doesn't hurt anything.

Thanks so much for your help!

G'day and welcome to the forums.

You do need to format it the drive so yes erase in Disk Utility and select Mac OS Extended (Journaled) as format. After that go intoPartition and ensure partition table under Options is GUID.

Then you have the choice of running Time Machine in System preferences or using cloning software. A cloned drive will be bootable in the event the internal drive fails.