Fluids (liquids and gases) form layers based on their density, which we find by dividing their mass (how much stuff there is and how heavy it is) by their volume (how much space they take up).

Gases (and indeed most substances) expand as they get hotter; the amount of stuff, and therefore mass, stays the same but the volume increases - therefore the density falls.

Hotter air is thus less dense than colder air and so it rises, causing the colder, more dense air to fall.

As you get higher in the atmosphere, there's more space to occupy (think concentric circles; they get bigger and bigger as the radius increases). With more space to occupy, again the density is going to decrease as the gases spread out. This also causes them to cool down (Boyle's Law, Charles' Law and the ideal gas law are a bit beyond a 5 year old though).

Also most of the sun's energy that comes to earth hits the surface (gas molecules are TINY so most of the sun's rays will miss them and some of the ones that do hit them bounce off) so the surface warms up far more than the atmosphere does. The surface then loses heat to the atmosphere. Since this heat starts at ground level, the air closest to the ground has more chance to absorb it and therefore the air closer to the ground will be hotter than air higher up.