This is copied and pasted from another thread (Oh dear lord- I'm becoming Revvy) but I don't want to have to type it all out again. Everything is as true as the first time I posted it:
You asked what the best thing for making good beer: #1 most important thing is FERMENTATION TEMPERATURE. The temperature in your beer can be as much as 10F above ambient, so you need to be actively cooling your beer. My climate is dry so I can get away with a fan pointed at a wet towel wrapped around my fermentor, but something fancier like a fermentation chamber would be nice if you feel like comitting the DIY time or spending a bit of cash. You can also do it on the cheap in high humidity with a muckbucket and frozen 2L bottles of water surrounding your beer. Regardless of what yeast say their "range" is on the package or instructions, ale yeasts should be low-mid 60s (F) for clean beer. Certain ales that take their major flavor from yeast, like hefeweizens or saisons can be fermented warmer, and certain strains like Nottingham english ale yeast prefer temps between 56-60F.
The next thing after that (fermentation temperatures are still #1. They'd be #0 if I had my way, though, it's that important) is pitching the correct number of healthy yeast into your beer. You'll have to some research on yeast starters, but they are super easy, and you don't need a stir plate contrary to popular belief, but you can get one if you like (it just means you can make less starter). Basically, you make a smallish beer and add your yeast to that. Grow them up to the correct amount (see mrmalty.com) for a day or two and then refrigerate them at least overnight to drop the yeast out of suspension. On brew day, pour off the clear liquid on top of your yeast, and warm the yeast up to pitching temperature (fermentation temperature) and add when ready.
Do those two things, and your beer will go from "meh" to "holy balls, that's good beer!"