Do a search here and in the fit forum, etc.. The short of it is think of your legs as a group of mechanical linkages, because they are.

If you lock your foot into a position on the pedals via the cleats that induce any kind of unnatural, or out of tolerance twisting motions, you can injure yourself.

Everyones physiology is unique therefore cleat placement follows suit and many things can impact the geometry in a negative fashion. Saddle position, cleat position is usually different for each foot, etc.. It's a whole different scenario when you anchor your feet.

Don't ignore any pain. Some types are to be expected, like muscle soreness from those muscles you haven't utilized nearly as much before (Quads!!) etc.. Be aware and pay attention to anything that doesn't feel right, don't ride through joint pain.

Work up slowly through your mileage, jumping considerably in a short timespan is just asking for inflammatory issues...

There's a lot to it, but it is all fairly simple. Err conservatively, ask questions.