Once you hit level 50, you can choose either Wetwork or Operator. Each class has 10 levels. So if you choose Wetwork, when you get to level 60, you can then choose Operator. 343 will be unlocking other specializations in the future and some Pre-orders have an unlock code for currently locked classes. You don't lose anything by picking a specialization.

I've got to say I'm a tad disappointed as the pre-order totally made it sound like you could use them right away, it should've stated in readable print that you had to level up first.