Your Apple ID is an account used to unify your obtained media/apps/content amongst your Apple devices. You do not need to create a new one for each new Apple machine you buy. Ideally, you'll keep one personal Apple ID for life. If you accidentally made a new ID, despite having all of your purchased content on an old ID, just don't use the new one. Log out in System Preferences, then log into your more fully-loaded Apple ID.