"I say yes."
Translation:Yo digo que sí.
Check out wordReference for conjugations: http://www.wordreference.com/conj/EsVerbs.aspx?v=decir
Dice is the third person present tense of decir, ie He/she/it says.
I say = Yo digo
Well, not always. For example, let's take the sentence "I say that she will take German 3 when I will take Spanish 2." This translates to "Yo digo que ella tomará Germán 2, cuando yo tomaré Español 2." The word "que" is used as a link between the two clauses. However, you should read more on the rules.
Diga, when used with yo, would be the subjunctive conjugation - here's the complete conjugation set: http://www.spanishdict.com/conjugate/decir
I'm not an expert with the subjunctive tense as of this message, but basically that tense is only used where uncertainty is involved, like, "I might say yes". Since you're definitely saying yes in the sentence given, digo has to be used.
The other diga is the formal imperative sense, a polite command. Commonly seen in Spanish lessons, when the course is telling its audience what to say.