**If you have never used a clicker with your dog before, it's important to build an association to the clicker/treat with your dog first. They must learn a "click" means a tasty treat is coming. So starting with some clicks and immediate treats, before clicking for any kind of specific behavior**
1. You must "capture" the sit naturally first. Choose a more calm and relaxed part of your day, and just be in a room with your dog naturally. When your dog chooses to sit on his own (and he eventually will) you must click the moment his bottom touches the ground and immediately pass him a treat (reward). You'll want to keep your treat hand and the treats very still as not to get your dog too excited he's receiving one before he does anything, which will cause unneeded distraction. You will repeat this exactly the same way multiple times until you see the dog's recognition that sitting means click and a treat.
2. Once your dog associated the click/reward with his sitting, you'll want to give the behavior a "cue". Most likely it will be you verbally saying "sit". Now you begin saying the word "sit" just as your dog looks like he is about to sit, well before his butt actually touches the ground. You're trying to model the cue as it will start to happen once he's fully trained. Then once he sits, it's click/reward time. If you give the cue and he does not end up actually sitting, you should say nothing and do nothing until he actually sits. Be patient. Once he sits, then click/reward. Since you'll be practicing this multiple times, when you reward, it's a good idea to toss the treat a bit away from your dog so he has to stand to go get it. Then he's in position to once again follow your cue.
3. Finally you transition to click/rewarding only the sits you ask for and not the ones he chooses to do himself.
Dogs need regular reinforcement to retain good training. Asking (cue) your dog to sit before meals for example, is good practice.
Here are a couple videos modeling this technique.