Sharepoint apps written in C#

Hi

I have a web part that is written in c# that hold sso for an outside vendor in our SP 2010 environment. It doesn’t access anything internally, it just creates the url to their site.

We have moved to sharepoint 2013 in the cloud and need to have a web part or app that holds the c# code. I don’t have access to a 2013 SP server with Visual Studio installed. How do I create an app with c#?

Any help would be appreciated!