Know what sort of soil you are working with. If you aren’t familiar with soil types, take a sample to your extension agent or garden center. They can help identify your soil type and help you choose how to amend it or whether or not it needs amendments.
Know your good garden pests and bad garden pests. There is no point in battling a beneficial insect. There is also no point in allowing a marauder to remain in your garden.
Learn to recognize weeds vs. your plantings. There is nothing more heartbeaking than tearing our your seedlings thinking they are weeds. Or leaving a weed to flourish thinking you planted it :p Been there. Done that.