how can I make a simple lowpass filter with this constraints?
check out the music-dsp source code archive here: http://www.musicdsp.org/showmany.php

I recommend a "Chamberlin" State-Variable Filter (which is 12dB/oct) which gives you other flavors (highpass, bandpass, etc) for free. If you cascade two of them you can get 24dB/oct, etc.