The concept of "on" is usually represented in context rather than with a specific sign. Instead of a separate sign for "on," you would often show the action of one thing being on another, or use spatial relationships to convey the idea.
However, if you want to show something being turned on, like a light or a machine, there's a specific motion:
If you're trying to indicate the position of one object in relation to another, you'd use the actual objects or representative handshapes and their positions to each other. For example, to show a book on a table, you might display one hand flat (representing the table) and then place your other hand, palm-down, onto the first hand (representing the book).
Always remember that ASL is a visual-spatial language. Many concepts, like prepositions, are conveyed through the use of space and the relative positioning of signs.
If you're looking to learn more sign language, check out our other ASL resources. We constantly update these resources to provide you with useful new signs you can use in your day-to-day life.