There is a couple of things you have to think about before installing a new OS on a disc (HDD).
First, check if the HDD is empty and doesnt contain any important information that you wanna save/take backup on before installing a new OS on it.
Second, you need to have a installation package(usually a iso-file) that you will need to use to install the new OS.
Third, Make a USB.flashdrive or CD bootable with the installation package(iso-files) and use it to boot up before booting windows.
If you havent done it before, i recommend using some guides on the internet before installing a new OS, since it is some important things to think about before.