I recommend that you top between the 4th and the 5th node. So starting at the bottom, and not counting the first two cotyledons (little leaves from sprout, which you can remove), count the number of sets of leaves (nodes) going up. Cut between nodes 4 and 5. If you cut the stalk cleanly you will have a successful top. If you cut through the 5th node and leave a bit of it alive, you will "FIM" it, which can also have weird and wonderful effects!
Enjoy! 😊