Part of being a successful pitcher is successfully fielding your position. Greg Maddux is a prime example. He's won numerous Gold Glove awards (it's baseball's highest honor for fielders) because he fields his position well. He's aware of the developing play around him. He knows what to do.
Part of Maddux's success at fielding is that he spends a lot of time and attention to being a quality fielder. It's important to him. He works on it in practice.
I teach the pitchers I work with to simplify things. If the hitter bunts the ball, for example, listen to your catcher. He can see the entire field. (You, on the other hand, are facing home plate, with your back to the field, so it's tougher for you to see the play develop behind you.)
It's your catcher's call on where you should throw the ball when you have to field your position.
Get to the ball quickly and anticipate a throw to the lead base. Set your feet as if you were going to throw to the lead base. For example, if there's a runner on first base, and the hitter bunts it, set yourself to throw to second base (the lead base in this situation). If your catcher tells you to throw to first base, it's easy to adjust your footwork and make a throw to first.
If you ever "fumble" the baseball or mishandle it, get an out where ever it's easiest to do so. (That's probably at first base.)