For starting out, I'd put my money on Dark or Illusion/Empathy Controller.
Both Dark and Illusion will provide pets and crowd control via sleeps/hold/fear making mobs less problematic for your teammates when they are shaking in their boots.
Empathy has PBAoE team buffs and single target buffs for your team. Depending on your team your may need to learn to buff on the go, rather than calling for folks to gather. If some are quick to dive in and move on, I'd recommend buffing with the PBAoE when you can hit the most, likely jumping into the middle of the brawl.
If you want more pew-pew and less cc, you can opt for an /Emp Corruptor which will give you ranged attacks rather than crowd control.
My very first character back on Legacy was Kinetics/Archery Defender but it's not the easiest to wield. Kinetics offers some lovely enemy debuffs that will also buff teammates that are near the target, but it does require an enemy target and you have to hit them.