Decir means to open your mouth and utter a phrase:

Quiero decirte una cosa – I want to tell you something
Quiero decirte que te quiero – I want to tell you that I love you

Both to say and to tell must be translated as decir. It is used always with a direct object (say something) and usually with an indirect object (say something to somebody). While in English it’s quite possible "I’m going to tell you (about something)", in Spanish you must use both direct and indirect objects, compare:

INCORRECTO: *Voy a decir ___ (no Direct Object nor Indirect Object)
INCORRECTO: *Voy a decirte ___  (no Direct Object)
CORRECTO: Voy a decírtelo = I’m going to say this to you

The expression decir que sí means "to accept", while decir que no means "to reject":

Me ofrecen un nuevo trabajo, y voy a decir que sí – They offer me a new job, and I will accept it

In Spanish, like in English, in order to show that we are willing to listen, we use

Dime / Dígame = Tell me

Hablar implies a conversation, an exchange of phrases. It corresponds to English to talk. It is usually used without a direct object, but with the preposition con (who you speak to) and de (topic of the conversation):

Voy a hablar con Lucía de política = I'm going to talk to Lucia about politics

Nevertheless, the topic of the conversation can also be expressed as a direct object:

Voy a hablarlo con Lucía = I'm going to talk about this with Lucia = Voy a hablar de este tema con Lucía

Instead of talking with (hablar con), one can talk to (hablar a):

¡Te estoy hablando (a ti)! = I’m talking to you! = ¡Me estoy dirigiendo a ti, no a otra persona!

"To speak a language" must also be translated as hablar un idioma:

Hablo español y francés = I speak Spanish and French

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