So, what is your mode of using IPv6? Does your ISP support IPv6? Do you want this global address globally accessible? Do you have a globally accessible IPv4 address?
If you do not need global accessibility, you may just assign address at random using 'ip' or 'ifconfig', but it doesn't seem to be what you want. You can place this in init scripts to be done automatically.
If you want to get IPv6 address from provider, you either need to have something like DHCP6 or just ask it and set this address as in the first case.
If you have an external IPv4, search the web for 6to4, 'sit0' is also a good search term (I do not have external IPv4, so I haven't used any of such solutions).
As a last resort, use tunnel broker. Personally I use go6.net (=freenet6.hexago.com, =freenet6.net), it is a free IPv6 tunnel broker which doesn't require to enter excessive personal data (e-mail and Name/Surname, that are not claimed to be verified; and all the data you send to Internet unencrypted will pass their servers - just as with ISP's). It will give you decent speed and a small globally accessible IPv6 subnet. It will go through most NAT's, though I need sometimes to help the keep-alive process by pinging freenet6.hexago.com (double NAT, though..).