Redirecting

Hi there,
I would like some help to understand the process of redirecting to another site. I have a site that I want when the user presses login to redirect to a platform I am using for services. The login and registration must go to another domain altogether. When I asked them for help they told me that I have to redirect to them using a sub-domain. As I am very new to all this and still in need of help, would someone be kind enough to help me as to how this can be done please ?