The question you need to consider is, which tab will users use more often. You want to make your site as easy to use as possible for the largest amount of users. Clicking on a tab is an extra step, so you want to set up your page in a way that the fewest possible users will need to perform the extra step.
Usually, there are more users who already have an account than users who need to create new accounts. For this reason, I would make Login the default active tab. I would also move the Login tab to the first position.
As an example, if you try to ask a question on StackOverflow before you are logged in, you are taken to the following screen:
As you can see, it defaults to the Log In tab since a user is more likely to already have an account.
Usually also, the Login tab has some sort of link directing users who don't yet have an account and can't login, to the register page. I would suggest adding such a button or link to your Login tab if you don't yet have one. Then, if you think a large portion of users will need to go to the register page, you can always style it to make it stand out.
(On an unrelated design note, having all your tabs black with the selected one teal makes it hard to tell which tab is the selected active one. I would suggest changing your unactive (currently Login) tabs to white instead of black. That will make your teal active tab stand out more.)