To choose a spec. other then spartan, reach SR50. Each spec. has 10 levels. As soon you choose one, you reach that first level for it. --- "You're dead if you aim only for kids. Adults are only kids grown up, anyway." - Walt Disney
The previous posters have already answered the main question, but in case you're wondering which spec to pick first, I personally recommend going for Pioneer first. The extra xp from Fast Track (the package you get when you max out Pioneer) might not seem like much at first, but it does add up over time, which saves you some time leveling up in the long run.
However, if you'd rather have something that gives you a spec that's useful during a match, I'd personally recommend either Wetwork (quieter footsteps, and it counters Promethean vision,) Operator (allows you to recover more quickly from EMPS+vehicles have better health regen,) or Rogue (gives you the edge in long range fights, and players can't just pelt you with precision weapon fire to counter Sniper weapons as easily.)
Lastly, you have to level up the separate specs to get their armor, package, etc.
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