Just start learning a programming language (which one doesn't make a great difference in this case), learn socket (internet connection) programming and a gui framework. But as I said start with the basics of a language first and then you will understand things better
A complete messenger will be a lot of work, but it's possible. For making an Android App you can use Java or a for a webapp HTML/CSS/javascript, other languages will work, too, but might be a bit harder for this purpose.
