In Cubase, you have a plug-in folder filled with VST synths and Effects.
You basically use a pull down menu to select which synth you're going to use.
Start a new midi track and set it's output to whatever synth you're using.
Then you play, it records the notes and velocities for you. Once you're finished you can use either the piano roll or score windows to edit the notes any way you want. Once you've got it sounding exactly the way you want, you export the MIDI track to an Audio track. (this turns the midi into an .aiff or .wav file)

Only thing you have to be aware of is latency. Check the specs on your sound card and get one as low as possible or you'll have a slight delay from when you press a key to when you hear the sound.

If ya have High Bandwidth, I've done a couple of tutorials on this subject, like this one...

http://www.guitartricks.com/lesson.php?input=7452&s_id=58

Right from starting with a drum loop/drum machine, adding the bass, punching it up with compression and amp simulation, adding keys, then adding guitar etc.. with all the mp3's and pics included.

Sampletank is easy. Just call up whatever instrument you want and play the midi file into it. You can do a movie score if ya want, just playing a couple of notes then exporting the midi file into a bunch of different instruments on SampleTank.
This one took me about 1/2 an hour just using drum loops, SampleTank and a couple of guitar tracks....all I did was play an A note, then exported to a bunch of different instruments picked at random.
Didn't listen to the finished song till I'd exported about 40 different things like tubas, sax, piano, bass, overdrive guitar.. then turned it up and it came out like this...

http://s93744050.onlinehome.us/SchmangeOnline/MP3Demos/DarkBattle007Demo.mp3