"Today I go to a youth gathering."
Translation:Azi eu merg la o adunare de tineret.
Compare the role of "to a" in these two sentences:
- I go to a youth gathering. = Eu merg la o adunare de tineret.
- I give food to a dog. = Eu dau mâncare unui câine.
In the first sentence, "to a" is used to express some place/destination, and is translated to "la o".
In the second sentence, "to a" is used to express the grammatical object receiving the food, and is translated to "unui". This is actually an example of the dative case in Romanian.
The sentence you wrote above does not make sense, because you have to express a destination, not a receiving object.