Translation:Where is dad? He is standing near the stadium.
Близко is an adverb. "Близко к стадиону" is "close to the stadium" and not very idiomatic, so we focus primarily on у, около, возле (with Genitive) and рядом с (with Instrumental).
Возле and около are largely interchangeable in the meaning of "near". Около is a bit more popular and is also used in the meaning "around, about" (approximately).
Около always takes genitive. You can look up стадион or any other word on En.OpenRussian.org or Ru.Wiktionary.org to find the declension. I like OpenRussian because most of the time, it has a pronunciation to go along with the word in the same speaker's voice (if not I turn to Forvo).
Here also is a table of prepositions, so you can see what case each preposition takes. http://russianlearn.com/grammar/category/table_of_prepositions