Daily broad-spectrum sunscreen use prevents measurable increases in skin aging, as shown in a randomized study of 4.5 years. Retinoids (particularly prescription tretinoin) reduce existing signs such as fine lines and pigmentation, but the effect is modest and disappears upon discontinuation. The combination of sunscreen as a foundation and a retinoid as a supplement is the best supported approach.
The most concrete thing you can do is apply broad-spectrum sunscreen every day. That sounds simple, but a randomised trial lasting 4.5 years showed that people who did so literally had no measurable increase in skin aging, while the group that used it at their own discretion did show deterioration. It is the only intervention in this area backed by that kind of evidence. Available over the counter, safe, and all you need to do is make sure you apply enough and do so consistently.
If you want to address existing signs of aging, retinoids are the option with the strongest foundation. Studies show genuine improvements in fine lines and sun-induced pigmentation. Over-the-counter retinol works, but less potently than tretinoin, which is available on prescription through your general practitioner or dermatologist. Things to bear in mind: irritation and dryness at first, and your skin becomes more sensitive to the sun, so combining it with sunscreen is then even more worthwhile. They are not recommended during pregnancy. The benefit is modest and disappears when you stop, so this is something for people who want to keep it up consistently.
The logical order is therefore: sunscreen as the foundation (preventing further damage), with a retinoid on top if you also want to reduce existing signs of aging.
Overview across multiple factors (3 research records, 4 sources). The strength of evidence differs per component -- read the answer for the nuance.