"I am at the beach."
Translation:Jestem na plaży.
It is female locative of „plaża”. There are multiple declination patterns, even within a single gender.
No. Generally "na" means "on" and "w" means "in". But then, prepositions differ between languages, so of course it's not that easy.
If you said "w plaży", I would probably imagine a person who is at least half-buried in sand and only some part is above.