I wish I’d learned a few useful knots earlier in life. Saves so much time when you know how and which one to use.
The ones I use the most are the square knot, taut line hitch and once a year the truckers hitch for tying down the Christmas tree to the top of the car.
Video guides are nice, but I prefer Grog's Knots. He even has an app for offline knot learning, say, when you're deep in the woods and it's raining hard and your tent's rain cover blows off into the lake and you thankfully brought a tarp and rope but don't know how to make one of those adjustable knots that you can just slip-tighten. You know, theoretically speaking.
On a side note and completely unrelated, bring one of those big grout sponges when you go camping. In addition to mopping up all the water in your tent, it makes a nice pillow if your inflatable pillow decides to run away in the night in a storm and go swimming in the lake.
For me, it's sheet bend, bowline, and round turn and two half hitches. I also tuck a lot of eye splices, but that's more just for fun; a bowline will work fine most of the time instead.
Man I'm an Eagle Scout and I forgot how to tie the basic knots already. It takes repetition and practice, I mostly use the square knot so that's the only one I probably remember.