Have you ever wondered what are the two most important things to do to produce good beer?
Honestly, everything is important. However, the two most important things are Sanitation and Temperature. If you maintain proper sanitation and temperature, your beer will turn out great every time.
Make sure that everything that will come in contact with cooled wort (wort less than about 180 degrees F) is properly cleaned and that it does not have anything stuck to it. If it is plastic, do your best to avoid scratching it. Anything stuck to it from previous brew days and scratches in the plastic can harbor bacteria that can contaminate your beer. Most plastics used for brewing can be cleaned with a regular yellow sponge. Hard scrubbing is typically unnecessary.
Once your equipment is clean, give it a good soak in a sanitizer (I recommend Starsan. It is a great no rinse sanitizer).
Temperature is important in all aspects of brewing. It is important to reach specific temperatures for extended periods of time when you are brewing partial mash or all grain beers. The correct temperature for the beer style and grain type is important for converting starches to sugars and getting the proper body of the beer. Too cold and you wont convert all the starches. Too hot and you risk extracting harsh flavors from the tannins in the grains.
When it comes to yeast, the temperature is just as important. If you pitch the yeast when the wort is still too hot, you risk shocking and/or killing the yeast. When yeast gets shocked or dies, it gives bacteria the opportunity to reproduce and consume the sugars in the wort and produce off flavors.
Once the yeast has been pitched into cool wort, it is important to maintain the recommended fermentation temperatures for the particular yeast strain and beer style. If the yeast is too warm, it will be stressed and produce fruity esters. If it is too cold, it will take longer to ferment. Longer fermentations are not a problem as long as you dont mind waiting a little longer before you drink your beer and esters are fine as long as that is what you are going for with your beer.
Remember. Sanitation and Temperature. Pay attention to those, and your beer will turn out great.