Dieting: When you creature eats something you want him to eat more of, reward him. Punish him if it's something bad. As for frequency, you will have to punish or reward before he downs the food in the first place.
Sleeping: GallopRider has the right idea for the most part. However, realize that once your creature starts sleeping in their pen every night, its VERY hard to get them to break the habit when fully grown. The only reason you should want your creature to sleep when grown up is to heal damage, and it would be very bad if they felt the need to cross the entire map to get to their pen. They may die along the way.
Because of this, I like to teach my creature to sleep where it stands, if possible.
You may want to use the Extreme Growth Map to get your creature to full size. If you use this map, though, you may want to remove all the rocks, mushrooms, and toys so your creature doesn't get distracted.
Also, because it is always night and the pen is so near, your creature will likely develop the habit mentioned above, and it can be hard to break. But I know it is possible because I have done so myself.