I don't understand the difference between some prepositons and nouns. for example- I came home from school at 2o'clock.

Saturday was a wonderful day.

pls explain me the difference.

'Nouns' and 'Prepositions' are two entirely different concepts. You can go through your study material to know about their differences. 

In the first sentence, 'at' is the preposition that denotes time. 'Home' and 'School' are common nouns.

In the second sentence, 'Saturday' is a proper noun. 'Day' is a common noun.