Go to the Solution Explorer (CTRL+W+S), then find your project. Right click on it and go to its properties. Find the Build tab. The top of the window will contain the active build configuration for your project. You can then change from debug (the default) to release.
By default, projects have two configurations: release and debug. You can make more, but first learn more about those two. The most important differences are explained in the question linked by Nacho in the comments. Good luck and happy codding.
edit: Web Site projects don't have the Release configuration available, but it makes no difference since they are not compiled. Web Application projects, on the other hand, do get compiled and have both configurations available.