The facetious answer is "post anything you want".

Realistically, you probably want some engagement, so post stuff relevant to your interests, and include hashtags so others will see.

Alternatively, if you want a little more structure, reply to posts you find interesting or have something to add to.

I tried Mastodon briefly, and I can safely say it isn't for me. I have a really hard time with the short posts, and the absolute lack of coherence between one post and the next. If you don't feel drawn to it, it may not be for you.