What to wear for a formal party?

You could make an appointment at department store with a personal shopper. They would be able to advise and help you choose something that suits your shape and style

You can’t go wrong with a simple, black cocktail dress and black heels or flats.

I like what @AshlynM said. Black always works, obviously your dress needs to be in the class of a cocktail dress. It can be short, slightly above the knee with a formal touch. Be sure to wear nice color blending shoes. You can also accessorize with some stone jewelry, a nice bold color. Make sure the color of your handbag/clutch matches your shoes. Formal evening pants can look stylish too. The pants must have some flow to them. They can be topped with a nice evening style top.

If it is formal, that means a long dress not a cocktail dress. Any department store or boutique could help this time of year. if budget is an issue, ray a consignment shop or thrift shop.

Formal can be to the knee, especially if you are young, but below the knee is better. If you go only to the knee a simple black dress made of a fabric that is formal is fine. You can go with another color too, if you do, it is better it be below the knee, stay with a solid color and no pastels unless it is at a beach or something you might get away with it. Beige or taupe is ok if you like lighter colors. The dress can have some beeding. Add some nice jewelry and a shoe with a heel you’re good to go. Your date, if you have one, should wear a dark suit, not a sportscoat.

I am assuming this is an evening party?

I also would ask other people what they are wearing, because you said formal party, not formal wedding, and I would want to make sure the term formal is being used correctly. The reason I ask is because if you are in a social circle that has formal parties all the time, I would guess you have dresses in your closet for formal.

This is an example of what to wear to a formal cocktail party

These are examples of what not to wear to a formal cocktail party

I’m in agreement with @JLeslie in that if you want to go above the knee then go black and go with a dressy fabric, something that looks elegant than cutesy. A full length gown can be most any color, again, more elegant than racy or frou frou.

I probably would not go full legnth to the floor for formal, but I would go as long as bottom of the calf. To the floor dresses are usually for black tie or white tie. In fact, I rarely see full length at black tie, but that might depend by region and community.