If, Then statements with a formula

I am trying to set up an If, Then statement in which I reference a cell. The
cell value can be a negative or positive number.

For example:

Column A is a number 1-999
Column B is a positive or negative or blank
Column C is a percentage if column B is negative only

Can anyone help set this formula up?

Thanks

http://spreadsheets.about.com/cs/excelfunction1/a/iffunction.htm

This site has pretty good explaination for the if, then statement. It gets into nested statements and things like that as well. It helped when I needed a little help in this same area.