I take it you’ve got a tower and a screen, which presumably you got separately. There are 3 common types of video connector: VGA, HDMI and DVI. Wikipedia has pictures of all 3 and (and various other ones) at https://en.wikipedia.org/wiki/File:Common_Display_Interfaces.png
See which type both the screen and the tower have, then you just need a cable with the same at both ends. If you haven’t got a suitable cable then you should be able to get one on eBay very easily - just search for (for example) “VGA cable”.