Next, you'll need a medicine ball. You are the rocket and the medicine ball is your fuel. Toss the medicine ball. You'll notice that as you shove the medicine ball forwards, you yourself lurch backwards. While you are pushing on the medicine ball, Newton's Third Law says that the medicine ball is also pushing on you.
Forces always come in pairs.
Of course, rockets work on more sophisticated principles than just tossing fuel out the back. And the rocket's exhaust nozzle has a narrowing so as to squirt the exhaust gasses out even faster, like putting your thumb over the end of a garden hose. Thus for example a Delta II rocket can send a 1800 kg payload into geosynchronous orbit, using about 200,000 kg of propellant. The total rocket at launch would have a mass of about 232,000 kg. If you could squirt something even faster out the back of the rocket, you could get more thrust with less fuel, and therefore send heavier payloads.