I don't think I ever use 'upon' except in fixed phrases, such as 'Once upon a time'.

'Upon' is a formal version of 'on', so you see it a little more in writing than conversation. You do still hear it on occasion, though: "The cat jumped upon the table" = "The cat jumped on the table" So any animal jumping on top of an object (like a table or desk) I sometimes interchange "upon" and "on" I see "upon" in writing a lot where it's being used as a more formal version on "on". "The politicians all agreed upon giving themselves a pay increase." "The committee called upon all interested parties to give evidence at the meeting" When conversation is a little more formal, you'd also say the above in conversation.

I got it, thaaaanks for your explanation.

Use "on" almost always. "upon" is rarely used. I sat on the bed. The cat jumped on the table. He drew on the paper. The word "upon" is typically used when referring to an inanimate object that you realize. It is typically used when describing your surroundings. It is used more often in literature and less in spoken English. There lies a vase upon the table. I saw a small figurine lying upon the windowsill