Should I tell students that it's my first time teaching the course?
Recommend not on the first day.

I'm actually in exactly this situation for the fall term, so I was prepping myself mentally for just this issue. In my experience, expressing any uncertainty about the course on the first day prompts almost instantaneous pleading/negotiation/argumentation over any and all stated procedures (parameters for assignments, grading, due dates, etc., etc.)

Now, this is partly influenced by the fact that I work at an urban community college where the student maturity level can be quite low for certain courses. If you work at a more high-powered institution then the environment might be different. But I still think it best to be authoritative at the start, gauge the environment on subsequent days, and then if you feel comfortable admitting the situation and/or asking for feedback later on, you'll be more confident about doing so.