Hey that's what I was doing all day yesterday!
This probably won't help you at all, but what really elevated my NPC game in this campaign was reading Harry Potter aloud to my daughter. There are about a million characters in those books, you have to be able to recall old voices and make new ones constantly. I try to make every named character sound a little different. It has greatly improved my ability to just whip out a character voice. I never used to be able to speak in character as a woman convincingly at all but I do it every session now.
Another thing that helps is having a team of players who are into it and speak in character when the character is talking. My last group was by and large pretty terrible at that which made it really hard to speak in character as a bunch of npcs. If your players are as green as you are, I'm willing to bet they don't do that very much. It takes some getting used to. If they don't do it at all, try to help them invent voices for their characters. Give out little xp rewards (or even just inspiration if you're playing 5e. I give out inspiration like candy) or maybe some sort of advantage in the next session to whoever put in the best performance.
Or maybe they aren't into the rp so much which is fine too, you gotta adapt for your group and what they want out of the game. If they aren't playing in character at all, I wouldn't worry too much about just describing what npcs do in third person. The challenge is when you have both types of player in one group, which happens almost always