Was that easy? Let's expand your source a little:
// My first BeepComp track! :) @G TEMPO=100 MASTERVOLUME=80 @1 O5L4 CDEFG~ @2 O4L4 EFGAB~ @D L4 KKKSSS
Now play this. You now hear a mix of three channels.
O followed by a single digit number sets the octave. O4 is the default octave where the middle C is, so O5 is just one above the default octave. L followed by a number ranging from 1 to 64 changes the note length. L4 sets up the note length to 1/4 of the measure length (quarter notes).
~ is quite useful. It elongates a note by the currently set note length. You can actually chain multiple ~s, for instance, B~~~.
Now we have @2 so commands after that goes to the second music channel. You can play up to 9 music channels simultaneously, so you can declare sections up to @9.
And you can use the drum channel by declaring @D. Write K for kick, S for snare, H for high-hat. In fact, you can use lower case k, s, h which will play a little quieter.
Note that you see some white spaces in above examples. You can insert any amount of white space between commands because they're ignored by the interpreter.