The first words you write are the first draft. Writing is thinking. You'll rarely know what exactly you want to say when you start writing.
The time you put into editing, reworking and refining turns your first draft into a second draft, and then into a third. If you keep refining it over days or weeks or even years, it eventually becomes something great.
Most writing mistakes are widespread, but good writers just get better at spotting them. Some things you'll learn to watch for are:
When you write something, you get very close to it. It is nearly impossible to distance yourself from it straight away to edit properly.
The longer you can leave a draft before editing, the better. Half an hour to two days is enough of a break to edit well. When you do edit, read your work out loud. You'll catch more problems and get a better feel for how everything flows.
It is more likely you've written too much than too little.
The rule for most writers is, "If in doubt, cut it." If a word, sentence, or paragraph isn't necessary, delete it. It will clarify what you're trying to say.
The beginning of anything you write is the most important part. If you don't catch someone's attention at the start, you won't hold it later.
You should spend a disproportionate amount of time working on the first few sentences, paragraphs, or pages.
The structure is what your writing hangs on.
A recommended how-to guide on writing good, clear English is “The Elements of Style” by William Strunk Jr. and E.B. White.