Wearing a stand alone SPF (not one in a moisturiser) of at least 30 every single day, winter and summer, is the best thing you can do. The best one is the one you'll wear every day. La Roche-posay is popular. This is what costs me the most because you need to use so much.
Use the gentlest cleansers possible and keep your face out of hot water. Keep your skin hydrated and moisturised.
Anti ageing products:
Retinol (form of vitamin A)
Vitamin C
Exfoliating acids
Peptides may have some benefit
Niacinamide (form of vitamin B) may have some benefit
Face yoga
Fillers and botox
There are a lot of crap overpriced products out there.
CeraVe does cheap and cheerful moisturisers and cleansers.
The best Vitamin C is Timeless for about £20. I'm using Garden of Wisdom because I like it and it's cheaper (£10) but I will try out the Timeless when I get around to it. Vitamin C fades pigmentation and improves collagen so fights sagging. It is an anti oxidant so it also fights free radicals which cause skin damage and aging. Vitamin E does this too.
La Roche-Posay do retinol products that people seem happy with. I'm using Garden of Wisdom at the moment, it was about a tenner. I'm going to switch to Paula's Choice 1% retinol though. The GoW is quite drying and seems to be increasing sensitivity. Retinol also increases sun sensitivity so you need proper sunscreen every day. How it works is it speeds up your skin cell production as far as I know but can be very drying.
Acids are AHA or BHA - AHA are glycolic and lactic and they increase sun sensitivity. Lactic is more gentle. BHA is salicylic acid and is more gentle and dissolves oil more readily. Lots of different products out there. Ones you use as a mask, ones you use as a toner and ones you leave on. Brands Alpha Gold I think, The Ordinary, Paula's Choice (I use Skin Perfecting liquid, about a tenner), Cosrx, and the Ferrari is Biologique Recherche. They remove dead skin cells so that fresher skin is revealed and give you a better more glowing complexion.
Peptides - The Ordinary, and other Deciem brands like Hylamide and Niod, M&S formula sleep cream, I think Paula's choice. Copper peptides and things called matrixyl and argireline. The jury is out on these but some people have had results.
Hyaluronic acid is a thing that helps your skin hold moisture. It's not an anti ageing thing by itself, but keeping your skin hydrated moisturised will make it look plumper and younger and of course it will be more healthy and less prone to ageing.
The skin care threads on here are absolutely brilliant if you want to learn how to take care of your skin on a budget.
Never start lots of things together. Introduce one thing at a time for about a month. If you start lots you won't know what is doing good or what is causing a reaction.
Face yoga - you tube. The muscles in your face sag, not just the skin. Exercising them can help.
I know very little about botox or fillers.