It's pretty easy to teach actually. Start out with a bit of food in your fist and put it in front of the puppy. Make sure he knows the food is there (try soemthing yummy like chicken). Also have another high value treat hidden in a treat pouch or on a nearby counter. He'll probably lick and nibble at your fist, trying to get to the food. Wait until he takes his nose away from his hand. The second this happens, click and give him a bit of the other, hidden treat. Repeat this (should only take a few times) until he starts to not put his nose to your closed fist at all.
Now you can start adding the command. Put your food-filled fist in front of him, and tell him to leave it. As soon as he does, click and treat. If he doesn't do it immediately, just calmly wait until he does. Repeat a few times until he's doing it pretty consisitently.
Once he is doing this fairly consistently, start opening your hand. If he tries to grab the food, calmly but quickly close your hand and pull it up and away from him. Do this swiftly and calmly so as not to excite him (as puppies are already super excitable!). After a couple seconds (if he jumps up on you, wait until he stops), put your hand back and again try opening it. Repeat this until he is not immediately grabbing for the food, and then start adding the command leave it (or whatever word/phrase you choose to use). Never allow him to recieve the object/food you were telling him to "leave it".
Now you can start practicing this by putting the food on the floor, on small tables, different surfaces, different environments, and different things to "leave it" (don't always use food once he knows the command!).
If this was confusing or you still can't figure something out, there are also many videos on YouTube to help you. I'm actually thinking about starting to make a puppy/dog training series.
If he's under 6 months old, remember that he is still a puppy so do not reprimand him if he doesn't perform the command. Also, make sure you can always enforce the command, for example, don't tell him to leave that skunk 50 feet away when he's off leash. And lastly, made training sessions short and fun for puppies. Keep them at 5 to 10 minutes long each and give lots of praise (but don't excite him during the session!). End each session on a good note and play with him afterward. Good luck!