These adverbs of location are indeclinable, which means that their form never changes. This makes them easy to remember.
Near the Speaker:
acá, aquí (here, over here)
At a Distance from the Speaker:
allá, ahí (over there, there)
You may notice that the preposition "para" (for, in order to) is often used before these adverbs of location. When it is, the meaning of the preposition changes to "in the direction of", so to speak.
For example, we can say "¡Ven acá!" (Come here), or "¡Ven para acá!" (Come over here!).