Being a rookie doesn't leave you with much in the way of "thinking" through the pitching process. You'll be given what and when to pitch by those who call themselves coaches. The irony of it all, is that a high percentage of this system is hit-n-miss at best. You'll get a sign for a fastball down and away ... only to have you send down range ... high and inside or a ball in the dirt bouncing off the catcher.
Roger gave you the dynamics of the environment and he covered it well. So to deal with everything in orbit that pertains to your question, let me answer it directly ... this way.
- You'll be going into a game depending on how reliable you are compared to others.
- You'll probably be put in to relieve another pitcher, and at that, depending how well or not your team is doing.
- You'll have to watch and take notes of why certain batters are placed in the batting order... and that has to account for just how good they are. Usually, your best hitters and those who can get on base, are in the top three or four spots in the order.
- Understand the "contact" points that every batter in the batting order brings to the plate. Usually, the first three and fourth batters make contact in front of the leading hip, middle of the batting order makes contact with the pitch off their belt buckle, and the bottom of the batting order usually makes contact off their back hip.
- Take careful note of any batter that has trouble with a certain location of a particular pitch. Some batters can't hit a pitch on the outside corner because of their foot placement in the box, while other couldn't his a breaking pitch to save their life.
- Now here's the steepest part of your learning curve - adjust.. adjust ... adjust. Adjust to the plate umpire's will and don't fight it. He/she is calling strikes and balls, not you, your catcher, your coaches. If you're not getting the inside, don't go there. DUHAAA...
- Take note on how really upset some batters get with a certain call. GO BACK THERE ALL DAY. In fact, you can send that pitch down range knowing it's not a strike, but the batter may not take the chance of getting a strike call based on his memory of that last time he failed at something similar.
There are tons of other things to consider, but for now learn as you go and don't get frustrated along the way. If this was easy to command, anybody could do. Pitching, doesn't work that way.