knee pain

[A___]

One of my biggest fear in life is that something happens to my body that prevents me from biking every day, and thought I've always had knee pain lately it's been getting much worse on rides of any distance. The pain is dull and behind the kneecap, and i'm realizing that it might be caused by cycling. Is there anything about bike fit / style I should look towards to try and fix this? I'm fairly inexperienced with bike sizing and geometry in general.
(for usage reference, I bike for commuting around the city, minimum 6 miles a day, only go on long joy rides maybe once a year)