TL;DR
Typically, each new theme starts on a Thursday when the old theme winners are announced.

Once the theme is announced Thursday, you can submit images before the next Tuesday (by midnight GMT-4).

You can submit 3 images (all in same post or 3 different posts if you like) but any images submitted for the theme more than 3 will be disregarded.

To participate you put "SteemitPhotoChallenge Entry" somewhere in the title of the post, and the first tag should be "steemitphotochallenge"
that should be all you need to do.

The themes are numbered (and 41 is the current one with winners being announced today)

More info on rules in the link sent by @tanyehwen...

so to get started, wait for the winners to be announced today (or tomorrow your time), check out what the next theme is, and submit your photos before next Tuesday (May 30) and you will be off and running.