Usually (but not always at all), you say "a" at the meaning of a short distance. "Á" is used in the sentences like "Eu dou um cachorro á sua mãe - Eu dou um cachorro A + A sua mãe", when article "a" and the preposition "a" are standing besides. Para is for a long distance but nobody will kill you, if you'll say "Vou a Portugal!", so don't worry about it.
(Sorry for my terrifying English, I'm not a native speaker).