Still need more info. What filter topology are you using? How many stages? Active/passive? Or is all that up to you to decide? Are you familiar with poles and zeroes on the complex plane?
The subject of filter design can involve some fairly heavy calculus. A starting point might be Wikipedia, http://en.wikipedia.org/wiki/Network_synthesis_filters
. There are links there to various types of filters: Butterworth, Chebyshev, etc. But Wikipedia mostly gives an encyclopedia-type treatment of the subject. To really make the choices needed to design one for a particular purpose, you may need something closer to a textbook.