first i would recommend finding out if this is a 32bit or 64bit computer from control panel in system information.
https://ubuntu-mate.org/download/
then i recommend downloading from this link the corresponding iso image.
get a USB stick that is atleast 2gb in size.
https://unetbootin.github.io/
you can use this program to install the ISO image on the usb stick.

i am not sure if you want to completely remove windows or have it along side with linux to test it but i have written for both a tutorial about it:
@maxsieg/guide-installing-linux-without-removing-windows