Go to "Player Management" > "View Roster" and select the player you want to power up or down. Press R or L until you see his "Pitching" or "Batting" attributes. The red area in all of the bars represents how skilled that player is in the category shown next to it. Press LEFT or RIGHT to increase or decrease the skill.
To do better in career mode as a position player (anything other than pitcher), put each of the first four hitting stats up as much as you can. Then use the remaining points on fielding and speed. Your player will probably start off in Triple-A, but he will get called up faster because of his higher batting attributes. Once you earn training points, add them all into the first four batting categories until they max out. After that, add all your remaining points into other batting categories, such as plate discipline and clutch hitting. Your player will become great in very little time.