Rule #1: Slow and steady! Try and integrate new products into your routine one-by-one, so you can actually tell what's doing what to your skin. My personal rule is only try one new product a month.
Rule #2: Don't create a routine you're not going to stick to. There's no point in buying 10 products if you know you're exhausted at the end of the day and will realistically only be bothered to do the bare minimum. There's nothing wrong with a small, simple routine — you can get wonderful skin benefits from just two or three products that are chock full of good ingredients.
Rule #3: Wear Sunscreen every day. There's truly no point in doing any other skin care if you're not regularly wearing sunscreen. People get caught up in the do's and don'ts of SPF — what if I'm inside all day? what if it's raining? etc. — but personally, I think this just gets confusing. My advice is to just make a habit of applying it every day. Making it a part of my daily ritual has ensured that I am always protected when I do spontaneously leave the house, or if it suddenly gets sunny and I want to go on a nice walk.