First, make sure you configured for Win32, and not Win64 (which is not supported yet), then, the easiest way is to compile from the x86 Native Tools Command Prompt for VS 2019, by typing
cmake --build . --config Release
(or --config Debug for the Debug mode) in the build directory containing the solution file (ROOT.sln).
Note that if you have a version of Visual Studio 2019 greater or equal to 16.8, ROOT will not compile, you will have to wait for the next version, with the upgrade of LLVM.
And if you search the forum, you will find several topics about ROOT on Windows, like for example this one: Compiling ROOT on Windows 10