I play SC1 like every day, and I'll help you with some definitions.
Microing - It's an act of moving forward, and backward on a small scale, to confuse the enemy, so you can take less damage, with more hits, it's like a way of tricking the system, but it takes practive.
APM - Actions per minute, it has to be the biggest variable in the game.
Microing small scale - microing units to kill other units/patrols.
Microing large scale - in a full on attack, still being able to micro. (putting weak ranged units in back, recycling.
Single player- this is a major part of the game, it is how you learn, and practice, before you get pwnt. It has a great storyline, so I advise playing SC1 and Brood war, to get to know the storyline.
after about 5 months into the game you should see some great UMS maps, prolly first one will be cat and mouse.
These are maps that are not counted toward your score, and are for fun, along with a totally different experience. (you could edit things so that stuff spawns, a zergling has 30 damage, and that it is called a flir flar. It's a total reconstruction of the game)
It will be fun, but the amount of learning you'll have to do is, ungodly, but think of it this way, it's all a mathematical equation. the miners you get will be specifically numbered for max potential, you building stuff in a certain area, at a certain time could be the matter between life and death. It's complex, but it's balanced, and that's why it's famous.