Your question is anything but specific so it took me some time to come up with a non specific answer.
You will obviously need to start in the beginning.
First you will need to familiarize yourself with MIDI and MIDI sequencing. There are different ways to go about that. The easiest is to type in What is MIDI? On Google and read as much as you can.
Once you’re familiar with most of the basic stuff that has to do with MIDI, you will need to learn how to use your X3. In order to learn without wondering in the dark the wise thing to do would be to read the manual. A lot of people skip that part.
Of coarse if you ever have more questions you can always post here or at the General Arranger section. We are always glad to help out.
A gentleman is one who never hurts anyone's feelings unintentionally.
- - - Oscar Wilde