newcomers has lots of resources about how to use Dreamwidth and where to find things you'll enjoy here. If you're coming from Tumblr, soc_puppet is posting tutorials for that angle.

The Follow Friday Master Post has lists of currently active communities by topic where you can look for some based on your interests. Many bloggers use recurring posts to pin down their key interests and maintain activity, which is also a good source of inspiration if you aren't sure what you want to post at this time.