Medium Trust is dead. We (the ASP.NET team) no longer support it going forward: our recent frameworks like SignalR, MVC 5, Web API 2, Friendly URLs, and the Katana stack all mandate the use of Full Trust. See https://stackoverflow.com/a/17218344/59641 for more information.
You have a few options at your disposal:
Contact your host and ask them to move your site to Full Trust. KB2698981 gives guidance to hosters telling them that they should move off of ASP.NET's partial trust modes and run everything in Full Trust with the proper OS-provided isolation levels.
Move to a different hoster that doesn't limit you to Medium Trust.
If you must use your existing hoster and they will not move you to Full Trust, use the older versions of the ASP.NET project templates (File -> New -> Project -> Visual C# -> Web -> Visual Studio 2012 -> { select app type }). Alternatively, you can start with the empty ASP.NET project template. These templates do not require Full Trust. However, take note that you'll be limiting the functionality available to your site, and you won't be able to install the latest NuGet packages provided by the ASP.NET team.