"There is soap on the table."
Translation:На столе мыло.
«Столе» is in the prepositional case, because the the soap is AT(prepositional) the location, not (going) TO(accusative), or FROM(genitive) the location.
«Мыло» is in the nominative case.
Some case rules regarding location can be found in the notes of this lesson: https://www.duolingo.com/skill/ru/Where-is-it%3F
"там" = There. Though it seems to me that "там" is only used at the end of sentences. For example, "the ball is there" = "Мяч там".
I am a Greenlandic kid who only speaks English and Greenlandic fluently. I may be incorrect about this...but in this duolingo course, I have yet to see "там" in the beginning of a sentence.