help for fft,ifft,fir design in c#

There are several ways to design FIR filter. Take a look:
"Sinc FIR filter design".
The main idea is to find the set of numbers which have a "filtering properties". All you need to do is to convolve the initial signal with the gotten "set of numbers". So, the job you need to do can be made in the time domain (without FFT and IFFT).

The most useful and handy tutorial for FIR filter design I have ever seen is here.