The Ditto Breeding Way
Probably the most simple thing to do, and is very helpful when wanting to complete the Pokedex. You don't have to search for extra Pokemon. How this works is you have to find a Pokemon (of any gender) that you want. Then, they will stay in the daycare, and the egg that is bred is the first evolution of the Pokemon that isn't Ditto. :3
Male-Female Breeding
The classic breeding option. Here, a male and female Pokemon that share the same egg group will be placed in the daycare. Their offspring's move and species will depend on the parents. The mother always passed on the species, and the nature. The offspring's moves would be the father's, and then if the mother had any of the moves in her egg move set, it would be passed down to the offspring.
Nature Inheriting
In Emerald, if the mother or Ditto is holding an Everstone, the mother's nature will be passed down to the offspring. This won't work with the father though. In Gen IV, this was changed to whoever spouse possesses the Everstone, there will be a 50% chance that the Everstone possessor's nature will be passed down to the offspring. In Black and White 2, the spouse holding the Everstone will always have their offspring have that nature.
To find out more, click here.
Hope I helped. :)
Source: Bulbapedia Pokemon Breeding