Currently Being ModeratedFeb 16, 2013 1:15 PM (in response to sadera610)
You don't: MobileMe was closed last July. Apple's iCloud service is a partial replacement; it requires Lion 10.7.2 as a minimum (Mountain Lion preferred) or iOS5 or 6.
You can open an iCloud account by signing in in System Preferences (or Settings)>iCloud and checking Mail and the other data types you want to sync.
More information about iCloud here: