Scorekeeping in baseball is a bit more involved than other sports and has a bit more of a learning curve. If you have been assigned to scorekeep, it is a good idea to shadow someone before your assignment to get an idea of what to do or ask for help when you need it.
The Scorekeeping 101 guide provides instructions on what to do and also the key codes. Another helpful resource is the Scorekeeping Cheat Sheet. The coach has the scorebook and usually carries around extra pens or pencils.
Below are videos that also do a good job of explaining how to keep score.
BASIC VIDEO ON HOW TO BASEBALL SCOREKEEP
MORE ADVANCED VIDEOS ON HOW TO BASEBALL SCOREKEEP