How can I filter my frequencies?

Hi,

I have a series of data (which are in time domain). I need to convert them in frequency domain by FFT method and after that filter my frequencies based on the maximum value of frequency and then convert them again in time domain. For doing FFT what I did is as follow:

X=xlsread('data100degree0.xlsx', 14); % I read my data from one excel sheet.

L=length(X);

dt=0.00075;

fs=1/dt;

t=(0:1:L-1)*dt;

out=fft(X,L)/L;

y=abs(out);

I do not know is it right or not and what should I do for the rest? (especially for filtering)

Thanks in advance for your help.