It depends on the church, at the one I used to go to skinny jeans and a cute top would've been fine. I'd say ask someone or go in a summer dress. As long as the dress isn't too short you should be okay. Maybe wear a light cardigan if it has thin straps.
Well in the church I go to the girls just dress up normally/how they want to. Some wear heels and skinny jeans with a cute top. Some come smart because they just came from work. Some just come casually. But I guess it depends on the type of church you go to and the type of people the church is for. My church is more for young people (young being anywhere between 16-29) and it is more of a social church that allows young people to get to know each other. So if it is more of a formal church then I guess you may have to go a little smarter and classy.
The last time I went to church it was for a wedding, so that alone probably made it a more formal attire than regular church, depending on the church. I'd ask around, but probably avoid going in jeans. Maybe some dress pants or a modest skirt and casual/dressy top, or a modest dress with sleeves? It really does depend on your church though. Maybe ask someone else who's going?
Most people wear a dress that goes to just above or at the knees. And the dresses also usually have straps or a little shrug/cardigan. But you can also wear nice pants or a skirt. You don't have to get terribly dressed up but looking respectable and nice is all that matters.