It doesn't matter how big a force you exert, or for how long. Any boulder can apply a large force, forever, for free. But when you exert a force and the thing you are pushing moves in the direction of that force, now you are transferring energy to it. It's a bit like paying someone a dollar: you can't go on doing this without replenishing your supply of money (or energy), and while there are many things that can happen to the dollar (or the energy) afterwards, it will always exist in the economy somewhere.
To calculate the amount of energy transferred, multiply the force you apply to the object, times the distance the object moves (in the direction of the force)
Energy Transferred = Force (Newtons) x Distance (meters)
The unit of energy calculated this way is called a "Joule" (pronounced "jewel").
1. To get a feeling for a Joule, hold something in your hand that weighs about 4.5 Newtons (1 pound). Now lift that object 0.22 meter (8 inches) straight up. You just gave the object 4.5 N x 0.22 m = 1 Joule of energy to move it upwards. The object moved in the direction you are lifting, so you have transferred energy to it.
2. Now let go of the 4.5 Newton object, letting it fall (move downwards) 0.22 m. What forces (in what directions) are acting on the object while it is falling? How much energy is trasferred to it? What forms does the energy take?
3. If you want to lift a 2 Newton object straight up 1 meter, how much energy will you have to give to it?
4. How much energy do you have to give a paper clip to lift it from the floor to as high as you can reach straight up?