It can depend on the game, but generally you have to groom and feed your pet regularly and win battles. Make sure you find out which food your Pokémon likes best, to build a bond.
Have your starter evolved all the way, then breed with Ditto. Trade that starter for the other starters. Or go on GTS and trade whatever the other person wants so...
Your Eeveelution doesn't have to have a Mega Evolution; none of the existing Eeveelutions have Mega Evolutions either. It's your Pokemon, so make it how you wish.
It depends on the car. For most new cars (newer than 2000 or so), you can search YouTube for "2005 Honda accord starter" or whatever your car is, and there's...
You can only obtain it via special event/mystery gift. However this game is no longer supported for events and thus Celebi is not obtainable without transferring it from an old...