Visual Studio will automatically convert your projects from VS2005 to the VS2012 format when you open the solution/project. Notice that an important change is that in VS2012 MSBuild is used instead of VSBuild for building the VC++ projects.
Depending on the projects you have you may run into some breaking changes that you have to resolve manually. Take a look at Lessons learned migrating to Visual Studio 2012 and .NET 4.5 to get an overview of possible problems and things to do.