Quick answer
Estoy de acuerdo.
the everyday version
Tienes razón.
the alternative — see below for when
Which one to use
The fixed phrase is estar de acuerdo. You say estoy de acuerdo, and to name the person you agree with you add con: estoy de acuerdo contigo.
There is no verb acordar that works here; acordar means to agree on a decision in a formal sense, and acordarse means to remember. For a softer agreement, tienes razón (“you’re right”) is very common, and exacto or totalmente work as one-word answers.
All the versions
Estoy de acuerdo.I agree. — the standard phrase.
Estoy de acuerdo contigo.I agree with you.
Tienes razón.You’re right.
Totalmente.Absolutely. — short and emphatic.
No estoy de acuerdo.I disagree.
Where you are matters
Identical across all regions; in Argentina and Uruguay the informal version is <b>estoy de acuerdo con vos</b>.
Check yourself
Which sentence means “I agree”?