あそこに　たなかさんが　います / There is Tanaka-san over there.

～が　います

means "there is ～" or "～ exist.". "が" is a subject marker, which name the word in front of "が" as a subject.

We learned the particle "は", which works in similar way to "が". It can be switched one another. However, "は" is not a subject marker, this is topic maker. "は" can be translated as "Speaking of ～" or "As for ~", which is very unique among other languages. I will tell you this further in another article.

～います

is only used for living creature, such as human, animal, but not plants(flowers, vegitables, tree, so on...).

たなかさんが　います

means "There is Tanaka-san.".

あそこ

means "over there".

あそこに

"に" is a particle for places, which can be translated as "at, in". So, it means "in the place over there".