Think of it this way:
If you lean against a wall you're imparting force onto the wall. Easy, right?
Now what would happen if you were to lean, but without a wall there? You would fall, because you're a clumsy dope
The reason you don't fall over when the wall is there is because the wall is pushing on you, keeping you from falling.
The wall doesn't give a stuff how much force you're putting on it, it's not going to move at all. However, the exact amount of force you're putting on the wall (the force of gravity, in this case), is exactly equal and opposite to the force imparted on you, and the wall pushes back with that exact amount of force
If newton's 3rd law were not true, you could run into a cement wall at speed and crash through it. The wall would go flying, but you wouldn't feel a thing because there's no reaction force