Sanitation (clean well with oxy or pbw and rinse, sanitize with starsan - don't rinse.) You cannot make good beer with poor sanitation.
Once you have that under control, Temp. control and yeast health. It is hard to make good beer without good yeast starter (for liquid yeast) and good temp. control (65ish for most ales - beer temp, not room temp.)
Start with good, tried and true recipes/kits and resist the urge to "change them" until you have brewed them at least once.
Rebrew the same beer multiple times - that is, ultimately, how you really learn to brew well. Change one variable at a time if you want to tweak a recipe.
Be organized. Have everything ready to roll. recipe, ingredients, clean equipment, etc.
Clean everything really well immediately after you are done using it - makes a huge difference in sanitation in the long run.
Don't get (too) drunk when brewing. Pretty much every stupid mistake I have ever made brewing beer came after 3 beers.