The good news is that you don't have to spend a lot of time and money to have good skin. Many times people add in extra steps for great looking skin, but in reality, sticking with the basics will keep your skin looking great.
But, if I had to say one step that you should start using today and continue always using, it would be a facial sunscreen. The sun causes the majority of the aging signs that everyone tries to avoid, and your right at a pivotal time where you can take steps to avoid those early signs of aging.
Seeing you try to avoid as many extra steps as possible, the good news for you is that you can get sun protection in with your moisturizer. Think of it as a two in one! Get a facial moisturizer with SPF 15 or higher and apply daily. If you are spending long times in the sun, apply again 20 minutes or so before you head outdoors.
Just by adding this step, your skin will look younger, longer.
If you want to know what my second choice would be, it would be an exfoliator. Exfoliating the dead skin cells off the layer of your skin will leave your skin looking fresh and new instead of old and drab. And, because you can get a daily exfoliating facial wash, which has gentle beads to work on your skin, you don't even need to add in another step. One of my favorite drugstore exfoliating facial wash's is Olay Pore Redefining Scrub (compare prices).