You don't want a hash; you want a MAC.
The scientific answer: you apply the MAC after encryption, not before. It is known as "encrypt-then-MAC" and it has a lot of benefits over doing things in reverse (aka "MAC-then-encrypt") or in parallel ("MAC-and-encrypt"). See this question for details.
The moral answer: don't do it. It is hard to make these things securely. You don't know enough to do it. Even people with a PhD in cryptography consider that they don't know enough to do it. When such a thing must be done, a cryptographer produces a tentative design and submits it to his peers, who scramble and try to break it for several years. Only survivors are deemed "apparently secure".
The practical answer: there are nice encryption modes which combine encryption and MAC for you; these schemes have survived the onslaught of enraged cryptographers. See GCM and EAX.