So I want to get into mario paint composing, but the problem is: I fail at it.

I've tried to make some good compositions, but when I try, it never sounds right.

I need some help from the awesome composers on this site (I'm talking about you, cat).

Sheet music. As long as you know how to read it, you can rule at MPC.

Nizmo's advice is good. Sheet music is a really good way to make a composition. To be honest, there are a lot of people that use MIDI files converted into sheet music to make them. You just need to download a MIDI of the song you want to MPC, and then run it through a program. I'm not sure what others use, but I use Noteworthy Composer. You just import the MIDI and BAM! Instant sheet music. One thing that can be tricky though is the clefs. Some of the the lines are sometimes written in bass clef, so you have to transpose, which really isn't that hard. All you have to do is bump said note from bass clef up 2 spaces. So D is on the middle line in bass clef, and B is for treble clef.

Well, composing by ear can be kind of tricky. Sure, pretty much anybody can pick up the main melody, but you have to either tear apart the music or listen really, really closely for any harmonies and countermelodies. Bass parts are usually able to be picked up pretty easily, along with percussion. It just takes a little training to be able to do it by ear. It's still useful to have a MIDI lying around just so you can get things if you absolutely have to.