Squeaky disc brake on a mountain bike

I have been biking for many years but I can't say I'm familiar with bike maintenance (I'm as lazy as you could imagine when it comes to maintenance.) Mountain bike and disc brake are especially new to me. My 2 month old bike started squeaking when I squeeze the brake. It sounded really scary and I worry about not having braking power when I need it the most: going downhill on narrow trails. I wonder if I did something wrong with brakes (misaligned it by accident?) but it turns out disc brakes just need more cleaning than the V-brake and C-brake that I'm used to. Thanks to this article. I didn't even use rubbing alcohol but plain water and it worked like a charm.