This is covered in the faq: 

- https://askubuntu.com/faq#howtoask
- https://askubuntu.com/questions/how-to-ask

These are both linked in the same page you ask a question (look on the right, by the formatting help, etc.) I've also made some videos to help:

- http://blip.tv/jorge-castro/how-to-ask-good-questionson-askubuntu-com-4927281
- http://blip.tv/jorge-castro/how-to-improve-your-questions-on-ask-ubuntu-4838345