'Outwards' is an adverb. It tells you about the direction of movement, so you should say 'The door opens outwards'. 'Outward' is an adjective. It describes a noun, for example 'an outward appearance'.

Yes, Su.Ki. is right, it is outwards. Although in America they also use outward as the 'same' adverb: "towards outside". Just in case you needed to know what they use there.

The door opens outwards