What Pokemon can you get from an egg and how on Pokemon Platinum?

Answer 1

The answer to this varies. For example, if you have a Ditto and a Pikachu in the day-care, you will eventually get a Pichu.

Much like other Pokemon games in the past, there is a day-care center in a specific town (in the Sinnoh region, it is called Solaceon Town). If you put two Pokemon in the same egg group* together in the day-care, the old man outside of the day-care will turn to the side. When you talk to him, he will say that your Pokemon laid an egg.

When you have an empty slot in your party, you can then carry your egg. For the egg to hatch, you must travel a certain amount of steps (varied by the different Pokemon). Eventually, you will suddenly stop, and a chat box will open up, saying, "... Oh?" and your egg will hatch.

*Egg groups are groups in which certain Pokemon can breed with each other. Ditto can breed with any other Pokemon. However, most Pokemon are in specific egg groups. Legendaries can not breed.

Link to egg groups: http://pokemonplatinum-version.blogspot.com/2008/10/breeding-guide.html