I always wear jeans, and I’m used to the way they hold my stomach in and make me look a bit slimmer. On the evening of the 6th I have to wear a dress for a formal event and I feel chubby wearing a dress now, lol. Even with leggings. My dress is just under the knee, so wearing jeans under it isn’t an option. I don’t have any money, so I can’t get dress pants or stomach control underwear.
I just turned 15, if age has to do with anything.
And my stomach does stick out and is rounded, soft and jiggles quite a bit if I walk around not wearing jeans. (If I sit down and put my jeans below my belly, it sticks out maybe 2 inches, when standing up with my jeans below my stomach it sticks out about 1 inch, maybe a bit more )
How can I feel comfortable in my dress? Or do anything to make myself LOOK slimmer, not necessarily lose weight?

Well, not knowing what you actually look like, I’m not sure how to help exactly. One thing I would suggest is wearing a dress with an empire waist that flows away from the body, but it sounds as if you already have the dress you’re going to wear. In that case, you could wear dressy cardigan with a nice belt over the dress. I love leggings so I would always pair those with dresses (or tights). They just generally make wearing a dress more comfortable I think. Just relax and try not to worry about it. Everyone is shaped differently. Hope this helps at all:)
I think you’re fine the way you are.