Is there a windows key on the Laptop if there is it will be on the bottom If not and you want a legit copy you will need to buy the OS of your choice . If your not so inclined to buy a key and copy you can always look at a less than legal route with Daz . Your best bet for an OS thats free is Linux ubuntu 15.10.
This will help with a windows install
http://windows.microsoft.com/en-ca/windows/installing-reinstalling-windows#1TC=windows-7
https://www.youtube.com/watch?v=lG0k7Z-HDuc
If you want to use Linux try this
https://www.youtube.com/watch?v=kL4DG31tgRM
In case you dont have a optical drive
https://www.youtube.com/watch?v=zvq1uJ76T0c
For windows USB install
https://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool
Hoope this helps