Learn Portuguese (Portugal): Eu digo feliz aniversário ao meu amigo.

To use the near future, you need the present tense of ir (to go) plus the infinitive of the main verb. The 'eu' form of ir is vou, and the infinitive of the main verb here is dizer. The sentence becomes: Eu vou dizer feliz aniversário ao meu amigo.

Why is it ao meu amigo instead of just a meu amigo?

In European Portuguese, it is standard to put a definite article ('the') before a possessive word like meu. You are literally saying 'to the my friend'. The preposition a (to) merges with the masculine article o (the) to form the contraction ao.

digo doesn't look like a regular verb ending. What is the infinitive?

digo is the 'I' (eu) form of the verb dizer (to say / to tell). It is a highly irregular verb, which is why it has a 'g' instead of a 'z' like its infinitive.

How would the end of the sentence change if I were speaking about a female friend?