What are you Agreeing "to" or "about"?
There are four ways in which agreement is commonly expressed in Spanish -- when considered from the ways in which English speakers use the verb to agree.
When you mean quite simply that you are in accord with someone's opinion on an issue or wish to just say you agree with someone, use the phrase estar de acuerdo. You can also use it to ask if someone agrees with you (or anyone else). In the following examples, note the various tenses used as well:
Creo que Juan y yo estamos de acuerdo (I think John and I agree).
Después de escuchar sus razones, creo que estaré de acuerdo con ellos (After hearing their reasons, I think I'll agree with them).
No estaba de acuerdo con su decisión (He didn't agree with their decision).
If you wish to state your consent, that you are going along with an idea or proposal, use estar conforme. Consider the following exchanges:
Es importante vender estas acciones inmediatamente (It's important to sell these stocks immediately).
Oye, ¿estás conforme? (Listen, are you in/do you agree [to this]?)
In business settings, there are two expressions used to express coming to agreements, as in a meeting of the minds to establish contracts or to covenant or agree to do something. One is convenir en -- think of convening a cordial meeting and you'll probably remember it easily. The other is quedar en -- which is used to express what people concluded and agreed to do about something. Both expressions often are followed by complementary infinitives. Consider these examples:
Los aficionados convinieron en comprar varios recuerdos de su equipo favorito (The fan club agreed to buy memorabilia of their favorite team).
Quedaron en volver a reunirse (They agreed to meet again).
¿En qué convenimos? (What are we agreeing to?)
Quedemos en esto: Vamos a vernos el jueves (Let's agree to this: We're going to meet on Thursday).