Every Monday we announce new weekly theme on a special page. Every theme has a unique tag assigned to it. For example, when the theme was Summer, you had to post a photo with "theme-summer" tag to enter the weekly theme challenge.
There's also a checkbox on the upload page that adds the theme tag automatically if you select it.
We post the rules for entering the challenge with every new weekly theme announcement. There's a Rules section. Please follow the instructions provided there.