The simple answer, Esther, is this - don't use it. You NEVER need to use the word 'upon'. Apart from a few antiquated set phrases such as 'Once upon a time' and 'Upon my word' which you have no reason to use, you can always use 'on' instead. If you see or hear the word 'upon', just think of it as meaning the same as 'on'. Otherwise, just forget about it. You'll never need to use it.

Use "up on" when the up is part of a phrasal verb, like "climb up", and think then of the "up" as belonging to the "climb" rather than to the "on" to keep it straight in your mind. "He climbed up on the ladder". Use "upon" when you want a single preposition: "He laid the book upon the lectern." (Although a simple "on" would work here too.)

Upon is a preposition which has two main uses: 1) It gives a time when some action begins: "Upon arriving at the house I saw the door was open." In this case it can mean 'When' 2) It is a slightly more formal way to say 'on': "Upon the table lay two books." Here it tells where something is- on the table. The word 'up' shows a direction of movement: "The rocket went up into the sky." It is common in speech to use the phrase 'up on': "Where are the cookies?" "They are up on the refrigerator." Here, 'up' is added to an already complete phrase to show the direction you will have to go to find the cookies.