Clean and modest is all you need. Jeans and a nice shirt won’t get you a second look, though some of the more traditional ones lean dressier (slacks, a dress, that kind of thing).
I kept wondering if I was dressing for God or for the people in the pews. I figured if God sees the heart, what I wear matters a lot less than why I was so worried about it in the first place.
If you want to be sure, check the church’s website or visit once to see what people wear (see this thread). A church that would turn someone away over clothing probably isn’t one you’d want to stick with.