Nokia Developer has THE community wiki for WP information - check out the great articles in Portal:Windows Phone. Getting started is pretty easy - first search on the wiki and on the Internet to confirm you're not duplicating a topic on the wiki or Dev Center (if you are then you should update the existing topic if at all possible or consider if you can create a better topic that adds value over existing material). The wiki is easy to use, but if you're stuck you can check out our Help and our wiki checklist.
So what to write about? I find that often questions answered on this discussion board are good for standard "How Tos" - like this one: How to detect if an app is running in Kid’s Corner
Other ideas are to update WP7.5 articles to WP8 - THIS LIST shows you some information on that.
Added: Check out articles that need to be updated. The template at the top of the page should state what the problem with the article is.
If you have other ideas, then feel free to add them as a response - a few more from me below:
* WP8 app compatibility issues (flashlight, FmRadio, etc).
*Working_with_Windows_Phone_devices_with_256_MB_of_memory could be renamed "Working with lowest memory Windows Phone Devices" and have section on limits of WP8 low memory ... or a new one for WP8 could be written.
If you can help with a suggestion here, thanks (and please let me know which)