The landscape of Open Education Resources can get very complicated very quickly.
If you're just starting out, the best strategy is to start by thinking about what you're looking for:
Do you need to meet a curriculum standard?
Looking for a full module (a few days to a few weeks) or supplementary material, or something to edit and remix for your purposes?
Does it need to work within your Google classroom/schoology/other Learning Management System?
Do you need an assessment component built in, like quizzes?
Just like searching for scholarly resources or a really good pizza place, what you need will determine where you start.
I've provided some samples in the box labeled: "Finding Open Education Resources" that have the ability to sort on many of these criteria.
When you find a source, it's good to evaluate it. Rubrics are pretty common, but can get dense very quickly. Try the 2-page summary of the Achieve.com rubric to get started.