Stitch markers! These little pieces of plastic genius are very convenient to have at hand when crocheting, especially when starting out and you haven't learned how to read your work yet (don't worry that comes with time and practice).
Stitch markers can be used when working in the round, for example for amigurumi there is no starting chain so they show when you've finished your row, just don't forget to move them up as you go.
When working back and forth, if you place a stitch marker in the top of your chain when starting each new row, you can see where your last stitch goes into.
This is a common problem with crochet, your last stitch will always be into the beginning chain from the previous row and it's hard to see, make your chains a little loose and place a stitch marker.
When making a granny square or anything square it's handy to place a marker in each corner, reminding you of where to make your next corner, also remember to move your marker each time.