You have two verbs, ir and venir, corresponding loosely to "to go" and "to come". The key is where the movement starts and ends (or its goal) and where you are right now. Your first example should use ir:
- Si voy a tu casa después de las 5, ¿estarás todavía ahí?
This is because you're not in the hearer's house right now, so you need to go there.
In the second example you should use venir:
- Vine aquí ayer y fue muy divertido.
This is because you're already in the place you're referring to (that's why you said "here" and not "there"), and you're describing your movement from somewhere else to this place. From this point of view you didn't go; you instead came.
The third example could use either ir or venir depending on context, but for the most part you would use ir:
- ¿Cuándo debería ir?
This is because "When should I come?" is not a question you'd normally ask if you were already in the place you're asking about; if that were the case, you'd rather ask "When should I come back here?" or "When should I come here next time?". If this "next time" has already been established by context, though, the question does make sense:
- ¿Cuándo debería venir?