Sounds very nice so far, keep working on it 
Changing the key is generally just moving all the notes up or down, with a few exceptions.Ã, In your piece, you could try moving everything up 5 notes for a while, and then maybe back down again later on.Ã, It sounds better as well if you change the melodies a bit for the new key, it adds more of a contrast.Ã, Here's a very quick example of changing the key and going back again.Ã, It changes at 0:47, and back at 1:12.Ã, Feel free to open up the MIDI and have a look if you need to.
edit

Changing the key is generally just moving all the notes up or down, with a few exceptions.Ã, In your piece, you could try moving everything up 5 notes for a while, and then maybe back down again later on.Ã, It sounds better as well if you change the melodies a bit for the new key, it adds more of a contrast.Ã, Here's a very quick example of changing the key and going back again.Ã, It changes at 0:47, and back at 1:12.Ã, Feel free to open up the MIDI and have a look if you need to.
edit