Whenever I was going to introduce a new concept in math, I would read a story that involved the new concept. The kids liked the story, and it was a way for me to assess who had prior knowledge about the new concept.
I compiled this list of books through the years. Some of the titles are older, but you may be able to find them in your school library.
Gather the books you want, add a sticky note to each book showing what concept it covers, and you will be ready every time you start to introduce something new. I have added the Common Core standards in case you need them for your lesson plans!