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.

I've been in this situation from both sides, and would say an emphatic "no" to telling students you've never done this before.

We know that the reality is that faculty are often thrust into a class without tons of experience at teaching it, but to the students, you're the expert. Starting off the class by essentially saying "I'm not an expert" will leave your students wondering what they're doing there. You shouldn't lie, of course, but it's important to the classroom environment that you show confidence in your ability. It's perfectly possible to learn from the class and be open to student feedback without proclaiming your lack of experience.

When I was in undergrad I had a summer course where the graduate student teaching it started off by telling us that it was her first time teaching. I felt bad for her, because she wasn't really great at it, but opening the class that way was like blood in the water to a classroom full of 19 year olds. Every fault was highlighted, every difficult exam was her fault, every difficult topic must have been difficult because she didn't know what she was doing. It made the experience worse for her and for the students, when it really didn't need to be that way.

Good luck!

Only if you're open to suggestions mid-semester.

Ask for suggestions on a half-sheet of paper about a third-of-the-way through the class. What worked so far? What hasn't?

If the class doesn't go well for whatever reason, and you told students you never taught it before, they will complain about you behind your back, and say you didn't prepare or you shouldn't have taught. They might complain to your advisor or the department head or someone else in the department.

If you don't want feedback, don't tell them.