The best you can do is get a market research book. There you’ll find extensive explanation of demand forecasting techniques.
There are qualitative and quantitative types of research, it all depends on your needs and on the information you can get. There is no magic formula, and the quantitative models are rather complicated to be explained here, there is much math and statistics involved.
Qualitatives (subjective):
- Experts panel (managers, distributors, etc.)
- Sales force estimate.
- Customers expectations survey.
- Delphi.
Quantitative (more objective results):
- Market research (Buying intention survey, Product/concept test, Market test)
- Temporal series (Random walk, Mobile medias (don’t know if it is the right translation, Exponential flattening, Series decomposition, Box-Jenkins, etc)
Get a book and take a good read 
Also go on and google [“demand forecasting” technique OR method] and I’m sure you’ll find plenty more information.
Regards and good luck with your project!