I am like 2 missions short of beating the entire campaign, for both the original Starcraft and for Brood War. There's roughly amost 10 missions per race per game, so almost 60 missions total.
I would google "Classic Starcraft Battlenet" and go to the Maps section and download the "Precursor" missions as a better introduction to the game.
Other than that, the campaign actually does a good job of ramping up the difficulty level, so if you pay attention, gradually try to gravitate toward greater hotkey use--but don't rush it, learn the tech-tree & unit capabilities first--then you should be fine up until the later missions.
What do you do at this point? I will let you in on a little secret. "+" and "-" key buttons are at this point your friend.
Slow the game speed down at crucial moments & key battles. It is true that if you ever play multiplayer, you will not be able to use this handicap, but do not worry about using it as a crutch if this is your first campaign playthrough, because you actually need to learn THOSE fundamentals of beating, of being ABLE to beat, the campaign FIRST, BEFORE you worry about competing at a multiplayer level, although of course except for those crucial moments, or at least as you build up in the mission, you want to be on Fastest speed until those clutch moments pop up.
Oh, and save often of course.
If you get bored with one race, feel free to play another meantime unless you INSIST on story continuity.