The locative case is only used with a few prepositions, most of which can also be used with other cases.
The one exception (preposition only with the locative case) is I think przy (by, near to, at)
w (meaning in or inside) when w means 'into' is takes the accusative
w mieście (in the city)
w miasto (into the city)
o uses the locative (meaning about) but can also take the accusative (
o ścianie (about the wall)
o ścianie (off of, against the wall)
na uses the locative to mean on, when na means onto it takes the accusative
na wyspie (on the island)
na wyspę (onto the island)
po uses the locative to mean 'after' or 'around' the meaning with the accusative is more like 'up to' or 'for' (
po kolacji (after supper)
These two links give examples of prepositions with the locative (they don't discuss the fact that four of them can also be used with the accusative
This chart gives a more thorough accounting of prepositions with the various cases
Contrary to gumishu, Polish isn't useless at all, only, it is something which requires ever so serious, and constant attention, near monastic focus, as I've said many times already. Poles are omnipresent beyond Poland, therefore, the language is more a presence in Europe and elsewhere now than at any time in history!
As far as Locative, location in Polish is tricky, mainly because the use of preposition can often be used in various cases.
However, the basic rules of "WHERE something is, e.g "Gdzie ona mieszka? - W miescie" etc. are pretty constant. Memorization of pattern usage though is still necessary, there's no way of getting around it:-)