Best way to design first order low and high pass filters in matlab?

I have a years worth of data for a given input signal (taken at 15minute intervals, so sample frequency is 1/900).

I now want to completely filter out all the amplitudes so that the fourier transform of the signal has an amplitude of 0 for all the frequencies.

I want to be able to easily play with the filter time constants of both a high pass and low pass (first order) filter, because my aim is to try and find out what time constants would best be suited to achieving my goal.

I am not sure what the best way to do this in matlab is though (not completely new to matlab, but it has been a long time since I used it'and I have not used simulink before). Is it better to try and make something in simulink or is it easier to do it in a 'traditional' manner?

If there were any examples or links you could recommend which could describe a way in which I could experiment with filter design easily, and also be able to observe the corresponding impact in the frequency domain it would be useful. I tried using simulink to do this just now, but then I got stuck with loading the input data from the workspace properly...