You have to have a pattern for the shaping and number of stitches,size etc. It's not a case of just knitting it pot luck.
There is software availble to be able to draw up patterns
http://www.greatknitdesigns.com/
But it is very complicated and you need to have a good knowledge of knitting. A knitwear designer would charge the earth for an individual design and writing the pattern. There are sites such as this
http://spincraftpatterns.com/index.php?cPath=5 0&osCsid=a431ae97cae3a01512dfb62d508947d5
but they are all generic patterns which you can adapt, the only bugbear being that it is an American site and they knit differently to us .
Another resort could be Sian Brown on this site
http://www.bhkc.co.uk/data/knitted_textile_art ists.htm
You can contact her and she may be able to advise you .
Good luck with your jumper .