Mew is an event-only Pokemon, so you can only get it when there is an active event (which come around only occasionally).
You can either trade for it on Pokemon GTS, use a cheat device, or deposit 999 pokemon on the wiiware "my pokemon ranch (which is only compatible with diamond, pearl, and platinum, not heart gold or soul silver) and make a trade. Keep in mind that pokemon like mew are not necessary to complete the pokedex officially. the same goes for Celebi, Jirachi, Deoxys, Phione, Manaphy, Darkrai, Shaymin & Arceus.