From your square dashboard you can go to the "Apps" section, then to App Marketplace and there are all the sites that are compatible with Square, once you find and locate the logo and name of your site just click on the button that says "get started" to link your site with Square. If it is a personal website you developed or someone else created for you you can call customer service and request to be contacted by the  API department and they will help you with the process to connect your site with Square.