select the ranges of frequencies to amply and suppress (such as in a percentage) in recorded .wav file

Hello, i am using fft to work as an equilizer. I am recording and saving the sound as .wav file and taking fft of the time domain signal. Now i wish to ask the users to define the rannge of frequency that they wish to amplify and suppress and change the amplitufe of fft respectively. I dont know how to proceed further and I am stuck at this part. Please help!