You should use soy when you are referring to something of permanence. For instance Soy de America. Soy alto. You use estoy for a feeling or location. Estoy en el cine. Estoy contento. Hope that helps.

Verb 'ser' indicates substance. Verb 'estar' indicates accident.

Soy means I am. Example: Yo soy alto= I am tall . Soy rapido= I am fast . Eso es lo que yo soy= that is what I am. Estoy is a little bit more complicated to explain. The first use is for a location. Estoy en el sur= I am in the south. Estoy bajo el agua= I am underwater. Mira esta foto ahi estoy yo= Look at this picture I'm there The second use is for feelings. state of mind etc. Example: Estoy enfermo= I am sick . Note that in this case if you use Soy enfermo that means I am a sick person like a crazy person but if you use Estoy that means i am sick or have a dissease. Estoy alto = I am tall. Note that is the same as Soy alto but in Soy alto is something that I am in this case is something I look like.Estoy contento = I am happy The third use is for an action, something I am doing. Example: Estoy lavando el carro= I am washing the car.Estoy estudiando= I am studying. Estoy siendo rapido= I am being fast. Note that in this case siendo(being) is an action that's why i can't use the word Soy as in Soy rapido wich means I am fast . I hope this can help you a little bit.