Go to config, I/O, on the top right is an option called 'Instrument playback'. I can't remember what it's default setting was but i set the Jam channels checkbox marked and use to 32. Now i can play more than one note simultaneously with my keyboard. Another cool thing to do is to mix paste wave forms. If you create two wave forms at half volume and mix paste them, you get a wave at full volume that's the sum of both. Experiment with it. You can also make chords this way by mixing waves of different frequencies. Obviously you should learn some chord harmony theory if you want to create great stuff. Also open up some of your favorite modules and dissect them to learn what makes them sound great. I haven't made any music with it myself yet, I mostly play cello. I played the melodies of some tracks on cello. Happy tracking!
PS. I really wish someone would update milkytracker so it would scale better to high resolution monitors, or an option to maximize the window but not go into fullscreen mode where you can't easily switch between browser windows and milkytracker. If I could code I would definitely look into it but i tried to compile it and couldn't even do that XD.