In episode 23 in season 1, I was confused on what Armins goal with asking help from Annie was. Was it to figure out if Annie was the FT? Or was it just to help Eren escape, and figuring if Annie was the FT could possibly be revealed along the way. If it's the first one, why did Armin take Annie underground? Also was there any significance as to why there were no civilians around?
This is explained in the next episode:
Was it to figure out if Annie was the FT?
As shown in the previous episodes, it was revealed that Armin already had a suspicion that Annie was the Female Titan, and revealed his theory to the rest of the company. Shortly after this, they had come up with a plan to trap Annie - hence them going to the underground tunnel.
If it's the first one, why did Armin take Annie underground?
This was a ruse to trap her in close quarters. With being in a confined placed, Annie wouldn't have been able to transform into her titan form without risking her own life in the process. As a warrior, her duty was to uphold and complete the mission at hand.
Also was there any significance as to why there were no civilians around?
This was a pre-determined route Armin and company had designed in their efforts to trap Annie. At one point, you see other scouts hiding behind counters, in homes, barrels, etc.... So no civilians were present because they were likely cleared out ahead of time to prevent casualties in the event something went wrong.