Pretty much the title, tonight I’ll be able to gain access to my first Specialisation and I have a few questions?
How does the choice system work/what can I chose?
What options do they all have? (in terms of unlocks and things)
Which is considered the most useful?
If anyone can give me a basic lowdown, I would greatly appreciate it, thank you! 
The choice systems works by just showing a list of specializations near the commendations section. You will just scroll down and select one, then it will ask you basically if you’re sure that’s the one you want.
I think you will get to choose between 2, Operator and Wetworks, although if you got a specializations code I’m not sure if others will be available already for you or not.
Once you choose one, you get some stuff at certain ranks/numbers. I think it gives you an emblem or an armor piece right away, but I can’t remember. Once you have moved up 10 numbers/ranks you will have completed that specialization, and get your perk along with all the armor pieces, like the Wheelman perk for Operator for example.
Then you will get to enlist in another specialization at that point, and do it again.
Personally I think the perk you get from Operator (Wheelman) is more useful than the one Wetworks gives you, but that’s just me. Wheelman makes it where if you’re shot with a plasma pistol while driving the vehicle just kind of slows/stalls for a second instead of coming to a dead stop. Its saved me quite a few times driving now, and got me some splatters too from people thinking they could stick me after emp’ing me. Wheelman also increases the health of your vehicle a little supposedly.
I think the Wetworks perk, whatever its called, makes you a little stealthier by making you quieter while sprinting, speeding up assassinations, and by making you less visible to someone using Promethean Vision.