The activity on the wiki has been slowly declining for a few years now. The K-ON! Movie (the last installment of the series) aired in 2011. I think our peak activity was around 2013-14. By now, most of the actual content of the wiki is complete, but there are a few projects that some members like vince are still working on.
If you feel like sticking around, start by just watching and enjoying the show! Afterwards, if you want to help out around the wiki, there's still a couple of things we could use help with.
This page is a good summary of the kinds of things that are still being worked on. However, a lot of these projects relate to content near the end of the series, so it might be difficult for you to contribute straight away, since you're new to k-on.
I think the best way to help out would be to watch and enjoy the show, and after each episode, check the wiki's article on it to make sure it's as correct as it can be!
I would just like to squeeze in and say that me and Stezie will look over your potential edits to correct them just in case :)