I have no idea why my text has no spaces in between?! Sorry!

There are a lot of uses but here are a few common situations Where 'At' normally refers to a specific location and 'on' normally refers to a position, or something you are physically on top of. So "I'm AT the shopping centre" "where abouts are you?" "I'm ON the third floor" "I'm AT home sitting ON the sofa" If used at the beginning of a sentence it can indicate time. "AT the moment, I am having dinner" "On wednesday, I'm going to the beach" ON can also sometimes be used when you are using a certain tool to engage in a particular activity. So "I'm AT home, I'm ON internet" "I'm AT the office but I'm ON the phone" ON can also be used when engaging in a 'general' activity. so "I'm ON holiday" "I'm ON a training course"

here is a link that should help you with all prepositions,hope it helps you :) http://esl.about.com/library/beginner/bleasypreps.htm

you can use on in several ways here are some examples. I have an appointment on Tuesday. or is the oven on?. here are some. examples for at. meet me at the bar or what time is it at?. examples for has. someone has to go to the store or Jessica has my computer. examples for had, I had a book or jenny had a dog but now she dosent. so it can be used in different ways but the main two ways are in question format or when you are talking about someone else. ;)