Start by practicing takeoffs and landings
These are the hardest maneuvers in flying and are where most accidents happen, so it makes sense that these would be where we should focus our time and energy.
Then work on slow flight (including stalls) and steep turns
These maneuvers will teach you how an airplane flies at low speeds/high angles of attack/high Gs, so they're important even though they're not particularly difficult or dangerous themselves.
Don't forget basic navigation
This is something most people never even think about because "it's just flying," but if your airplane ever has any kind of problem, then knowing how to navigate around obstacles can literally save your life, so this is another important skill set worth practicing regularly.
Finally, it's time for Cross-country flying
This one is more for fun than anything else since most pilots rarely fly cross-country except for recreational purposes, but it's still worth learning at some point since it teaches us how our airplanes work when we're far away from home base (and gives us more practice navigating around obstacles).
So those are the basics - takeoff, landing, slow flight, steep turns, basic navigation, and cross-country flying. All of which can be practiced relatively cheaply with an instructor at pretty much any airport in the country (just call up the airport manager or call AOPA or check out Fly'n'Buy).