To catch Jirachi you have to get 5 stars next to your name on your trainer card.To do that you have to defeat the elite 4,win all five contests and have their portraits drawn,win 50 battles at battle tower,complete your pokedex,and defeat EVERY trainer in the game.Then go to the space center in Modseep and talk to everyone in there. One of them will tell you the number of the flight.If it is flight 100 he may let you on the rocket and you will encounter Jirachi in space.Or you will just encounter him in the wild after you get 5 stars but try the rocket thing first.