asp.net how do I use it?

I have had an outstanding task for a couple of years, waiting for a solution.

I have two websites. A public access site and a retail site.
The retail site needs to be logged into to see what is for sale and purchased.

My hope is to have a link in the public site that auto logs the potential customer into my retail site.

The retail site people say the code is asp. net.
However I am a PC tech with no programming experience to fall back on.
I think I have the code saved but have no idea what to do with it.

If you can help, that would be just great.