You need to first determine what are the frequencies you want to keep and what are the frequencies you want to filter out. You can then use tools to help you design the filter.
Any DSP book will talk about the theory. For tools, take a look at the following documentation
doc fdatool
doc fdesign
Here is an example
x = rand(1024,1);
hd = design(fdesign.lowpass);
y = filter(hd,x);
