To use vst instruments, go to devices, vst instuments, click on a blank slot and cubase will show you the vsti's you have. Click to choose one.
Some instruments come with there own kboard on the gui so you can try sounds out, some don't.
To play/record them you need an external midi keyboard plugged in.
Or you could import a midi file (File>import midi file), and then put your vsti in the midi output slot for that track.
Ypu can use the midi editor to input/edit the midi file.
As paul999 said cubase comes with a very comprehensive help function, which does provide all this kind of info, its really mostly a question of taking the the time to read through, to be honest.
Good Luck