Coilless safety pins (pictured on the left) and split ring stitch markers both work.
You can even snip a short length of yarn and fish that through to mark something. Since you likely know where to find yarn, it can be an easy way to mark something in a pinch. And, it’s super-cheap too!
Be sure to have a sharp pair of scissors and make sure that you save them just for your crafting…
This is just a sampling of what’s in our Crochet 101 PDF!