What have you actually studied so far?

Analogue filter design is a very long-established field - the basic principles haven't changed much since the days of thermionic valves (tubes).


In fact, Mr Butterworth described his filter design in 1930 - see: http://en.wikipedia.org/wiki/Butterworth_filter

There are very many books and articles on the subject.

These days, the major linear chipmakers have extensive design resources; eg
Linear Technology: http://www.linear.com/designtools/software/#Filter
National Semiconductor: http://www.national.com/analog/am...ch_filters
Analog Devices: http://www.analog.com/en/amplif...s/fca.html

Of course, you need to understand the principles before diving-in to an automated tool...