Time, grass seed, fertilizer, and lots of water. But mostly time. I started down this journey last fall/late summer and have seen a lot of progress but there's a long way to go still. I'm hoping to have it where I want it in a couple of years.
Look for local nurseries/grass places for advice; around here there's one that sells a yearly package of seed and various fertilizers, and people seem to like it. They'll also be able to give you advice on what type of grass to plant, specific to your climate and the amount of sun/shade in your yard.
You'll probably want to do some verticutting/dethatching if it's been a while. I've been seeing ads for small electric models from SunJoe and they look fine, but if you have a Home Depot close by, and a way to get it home, you can rent a much more capable device for much cheaper. It will seem counter-productive as it will thin your lawn for a bit, but it is necessary to get good ground contact for the overseeding you'll do immediately after.
If you don't have in-ground irrigation, invest in some water hoses and sprinklers - you'll be using them a lot. Depending on what part of the world you're in, near daily watering is likely to become a ritual later in the summer.
If you have absolutely bare spots, put down some starter fertilizer, a handful of seed, cover it with a thin layer of sphagnum or peat moss, and keep it damp. Don't soak it, but you'll want to go over it daily when it's not raining.
For dandelions and other weeds that are tough to pull, I recommend a weed pulling tool. They're $10-15 on Amazon and sort of fun to use once you get the hang of it. There's something satisfying about pulling a dandelion and bringing up an entire 8" root. Some people even use these for tea but I haven't tried it...
