How does installing a new OS work anyway?
Download it, install it and restart then it's done?
Depends if you wanna just erase everything on the drive and start fresh or if you want 2 different OS installed or if you want to install a single OS and keep all your files.
If you want everything gone, put it on a DVD or Flash Drive, (assuming you didn't buy the OS) format the harddrive it's going on, put the disc/drive in and start up the computer and follow the onscreen prompts.
If you want 2 OS, create a partition big enough to hold the new OS (could find the room you need easily by googling it) and install it on the new partition.
If you want a single OS and all your files kept on the computer, create a partition big enough to hold all the files you want and move them there, format the other partition(s) then follow the standard install procedure.