FIR filter designing in Matlab
Have a look at the examples for fir1, fir2, or firpm. The first two are straightforward, while firpm is a little more complicated as it derives the minimum filter length needed to match the design criteria, but it's handy to know.

For a simple lowpass filter this should be enough: Feel free to play around with N or look into using firpm to get the minimum order of N.

fc = 100; fs = 1024; N = 256;
b = fir1(N, 2*fc/fs);
freqz(b, 1, 65536, fs)