Before using a charm you need to:
1) check in the charm's description if it is of any use...
2) check the charm's cost and see if you can pay for it (your essence pools and willpower scores are in "stats").
3) check the type of the charm and see if it could be used (types of charms explained in "rules and precisions").
4) be sure that your character does not need to activate another charm the same turn (a dodge charm to save his/her life for example).
Then, once you have check all of these, you tell the Game Master in an OG add on to your post which charm you want to use, with which pool you want to use to pay for it and the number of motes you are going to invest (if required) and how many times you are going to use this charm on this turn (if required).
I think these are simple rules but they can make the difference between a dead character and a living one : )