You can engage them, or you can lose them. Guidance departments do a brisk business during the first week of school changing schedules. The first day, the first week, is critical for the beginning Latin student. Many of your students are already worried that they aren't smart enough to take this class. They are wondering if they should have taken Spanish. They have a lot of friends in Spanish. Perhaps they have an older brother or sister who took Latin. But he or she was the smart one as they are sure all their teachers already know.
If you want to thin out your class, you should list all the cases in order and then put the first declension endings on the board. Or you can explain that there are three genders in Latin that have different endings except when two of them have the same endings in the third declension and then put all those endings on the board. Either way, if you begin this way on the first day or the first week, you will likely convince some of them that their initial fears were correct. The line at guidance will get longer.
Do not begin by reading the syllabus, the class rules, the weights for formative vs. summative assessments, late policy blah, blah blah. Your students have already heard this at least once and maybe four times depending on what hour in the day you see them. No one is listening. Students rarely get into trouble in the first week anyway and you can be sure that any student who does, would not have been deterred by a recitation of the class rules. Yes, this is necessary information but save it for the second week.
I suggest an alternative. I constructed this little "Survey" for the first day of Latin ever and it has served me well. In the drive are two versions, a middle school and a high school version. I explain that it's not real survey but a way to talk about all the cool things that are Latin. It will not be graded and it will not be collected. Guessing is encouraged. They take it and then we discuss the answers. The whole process takes 50 minutes. There will be some giggling. At some point, a few students will reveal shyly to you that they read mythology on their own or that they have seen Gladiator on TBS three times. They will leave relieved. So far, Latin is okay. Later, they will tell someone about that crazy Roman emperor who made his horse president of Rome. Oh, and the Latin teacher is nuts too.
About that kid...
Every year, I have one or two students who actively or passively work against me on the first day. You probably have this kid too. He talks to his friends around him, yells loudly that he "doesn't want to be here" or just calls out random remarks or obvious wrong answers to questions. It's tempting to yell at that kid, send him or her out of the room etc. If you can at all help it - Don't. Any kid working that hard on the first day to get on your nerves WANTS that to happen. Why? Because then they can label you as "mean" and give themselves permission not to try. This kid doesn't hate you - he doesn't know you. Their behavior isn't about you - it's about their own discomfort. Remain calm and address their off topic comments briefly but seriously. For example, "No, I don't know if aliens visited ancient Rome but I think that would make a great short story." After class, check in with the kid. "Are you okay? I'm sorry that you didn't get into Spanish/Chinese/ etc. I want you to be comfortable in here." Often this knocks them off balance because usually they were expecting a reprimand or detention. What I find is that over the course of a few days, their disruptions decrease and they settle down. Of course if it doesn't, then you need to address it with the usual fare of disciplinary measures but usually when treated with a healthy dose of kindness and good humor, the problem stops on its own.