Hi there,
your best bet is to install Windows Marketplace and try and get your google maps through it (
http://marketplace.windowsphone.com/Default.aspx).
If you downloaded a .cab file directly to your phone, you will need to make note of where you placed the file, then use your File Explorer application to get to it, tap on it and it will install.
If it's a .exe file, you need to connect your phone to the computer via active sync/wmdc and then do the exe install.
It will soon become second nature to you.
