You can buy fashion but you cannot buy style. When attending a cocktail party, you need to be highly fashionable. A cocktail party is a social event. It is an event where you should dress to kill.
There might be a dress code. If that is the case, you should follow it to avoid being the outcast. If there is no dress code, you will have the freedom to choose what to wear. However, you need to keep in mind the tips for dressing for a party. Of course, you should look informal.
If you do not have a fashionable cocktail dress, you can opt for cocktail dress hire. There are many options available. You need to find a dress that will properly highlight your curves. You need a dress that will match well with your jewelry, skin color, and hairstyle.