Hola Kathy. Like you, I'm just learning, but I think I can answer your question. If you were speaking to a friend, you would say "Please open the door" this way: *Abre la puerta, por favor*. "Abre" is the imperative form, the form you would use to tell or ask someone to do something. If you were speaking to someone you didn't know very well or someone to whom you wanted to show respect (someone you would address as "usted" rather than tú), you would say "Abra" instead of "Abre." Both are imperative forms of the verb abrir, to open.
If you want to ask your friend "Do you open the door?" you would say *¿Abres la puerta?* or *¿Abres tú la puerta?* If you were speaking to someone you would address as usted, you would say *¿Abre usted la puerta?*
Note that the tú form of the imperative is the same as the 3rd-person singular present tense. That may seem confusing at first, but normally the context will make the meaning clear.
I hope this helps. And if I'm wrong, I hope someone will correct me.