When writing the first draft, the most important thing is to actually get stuff onto the screen. For me, that means forcing myself to write at least a page per day regardless of the quality of what I write. The first draft by definition is the worst draft which means you'll have to rewrite parts of it anyway, but first you need a completed story to edit in the first place.
When typing up the first draft, it doesn't matter if what I type is grammatically correct or not, or if the characters are in a scene which isn't necessary, or if I'm just piling in adjectives and adverbs to fill space. All of those problems can (and should) be corrected when editing the draft later. Before you do any serious editing or agonise over syntax or minor plot details, you should work on finishing your first draft first.