As has been pointed out, your question is too broad to be answered in a forum. It's like asking "Can someone please tell me how to fly a Boeing 747?" in a travel forum. You should read a book on expert systems. That will be much more helpful than just Googling, because you (currently) don't know what you don't know.
I highly recommend Rule Based Expert Systems by Buchanan & Shortliffe. Having developed expert systems for 8 years and having worked in the field of AI since 1987, I can assure you this is an excellent book (and an easy read), expecially for the problem that you're trying to solve. One of the authors (at Columbia University) has made a free PDF available here:
Rule Based Expert Systems by Buchanan & Shortliffe
Getting back to your problem, I expect you will want to build a backward chaining (diagnostic) expert system. This gives you the advantage of being able to ask the expert system:
- Why it's asking you a specific question
- How it arrived at a conclusion
Good luck in your efforts! I hope you have as much fun building your solution as I've had working on expert systems all these years.