1. Mass: The more massive an object is, the more momentum it will have at a given velocity. Think of a bowling ball versus a tennis ball. They both moving at the same speed will have very different momentums because the bowling ball has much more mass.
2. Velocity: The faster an object is moving, the more momentum it has. Think of a car slowly rolling down a hill versus a car speeding down the highway. They have the same mass, but the car moving faster has much more momentum.
In short, momentum is a measure of an object's inertia in motion, and it depends on both how much "stuff" it has (mass) and how fast it's moving (velocity).
