First week as a sub, and a kid told me I 'have a lot to brush up on,' I need help

I am a new substitute teacher at a middle school.

I'm 5'1. Lots of kids are taller than me. One tall kid already came up to me and said I'm short. I nervously told him, "Yeah.."

Not just that, but I'm also having a hard time acting like an adult. I think I'm identifying with the kids too much. For example, this is my first week monitoring student behaviors during lunch time. Lots of kids have already started testing their boundaries, joking around just to see how I would react. I have observed that many of them steal food, accuse their friends of cursing, etc.

Today was quite the day. One eighth grade jokester kid said that his neighbor was F bombing and cursing, and how it was against the rules. I personally didn't see it, but knowing jokester loves to joke and mess on authority to see their reactions, I just went with it. I told the person he was accusing, "Hey, stop cursing! X, Y, did you see him cursing?" in a smiley manner. But the accused kid looked very distressed, and said that Jokester was stealing his lunch. I asked the kids around him if it was true that Jokester was stealing his food, but everyone confirmed that Jokester did not steal food. The kid still looked distressed, so I asked him if he wanted me to call the school aide. The accused kid said, "yeah, sure." But everyone confirmed that Jokester hadn't actually stolen anything either — so at that point I had two accusations and no idea what had really happened. In the end, the school aide handled the behavior.

When I went back on lunch duty, the accused kid talked to me as he was eating his lunch. He said, "You must be new, you have a lot to brush up on as a lunch monitor." He said I needed to follow the consequence system and report stealing to admin immediately. I felt ashamed, acted immaturely, and like I let this kid down.

This being my first week, I'm having a hard time managing student behaviors. I don't know how to balance being fun with seriously reinforcing the rules. I find the kids tune me out when I'm too serious with them and don't follow along with their jokes. But if I go along with their jokes, I feel like I'm identifying the kids and not being the adult in the room. I just don't know how to... act? Experienced subs, do you have any advice for me? Is there anyone who can relate who can also give advice on this matter? I just don't wanna get taken advantage of in the classroom. I know kids can smell a nervous sub, and I'm already anxious.