The first step is always to check the party invitation. Usually the invitation will give you a clue into the formality of the event or even the dress code such as white tie event or a black tie event etc.
Try on a lot of different dresses to find the right style and fit for you. You may like a dress for the style or color but it may not be the right fit for you. Pay close attention to the fit of each garment for the formal event. If there is any pulling, bunching or creasing, then go up a size. Take a friend with you for moral support and a second opinion.
Colors play an important role in selecting formal party dresses for women. For example, for a fancy wedding, you may want to stick with cocktail dresses of basic or understated colors. Black, navy and other neutral colors are great to wear to a wedding. For a semi-formal event, you have more freedom to choose from a wider array of dress colors. For these events, you can wear pinks, bright blues, or other fun colors.