I am

Is there a rule when to use soy or estoy since they both mean I am?

Both are from the verbs Ser and Estar and both mean to be. They are however used in different situations . Generally speaking, Ser is used in descibing permanent situations and Estar is used in describing temporary situations. An example I use is in describing an apple. The apple is green. You need to decide if the apple is green because of its variety (Ser) or is it green because of being unripe (Estar). Another is the coffee is hot. Use Ser because it is supposed to be hot. The coffee is cold. Use Estar because it is supposed to be hot. In other words Ser is used to descibe an inherant condition. Now, having said this, there are rules that state that you should use one or the other depending on what you are describing, for example..occupation or location etc. These can be found in most Spanish grammar references. I hope this helps.