You Must Wear a WaistcoatAlso known as a vest, a waistcoat is a common component of a three-piece suit. However, that doesn't necessarily mean that you have to wear one. For most occasions, it's perfectly fine to omit a waistcoat from your outfit. You'll still need to wear the other components of a suit, including a suit jacket with trousers, but you don't have to wear a waistcoat.
Your Suit Jacket Must Match Your TrousersSuits are typically sold as sets consisting of a suit jacket with matching trousers. If the suit jacket is black, the trousers will likely be black as well. But contrary to popular belief, your suit jacket doesn't have to match your trousers. You can wear trousers in a completely different color than your suit jacket, assuming the two colors flow cohesively with each other. If the color of your suit jacket clashes with the color of your trousers, on the other hand, you should explore other outfit options.
You Can't Wear a Sweater With a SuitDuring the cold winter months, many men avoid wearing suits because they believe that they can't wear a sweater with them. Even during formal events and occasions, however, you can wear a sweater with your suit. Simply wear the sweater over your dress shirt and pop up your shirt collar so that it hangs over the top of the sweater. Depending on the size of your suit jacket, may be able to wear it over your sweater for added warmth.
Your Socks Don't Matter Because No One Sees ThemWhen putting together a formal event, you shouldn't ignore the color and design of your socks. If you are standing or walking, no one will be able to see your dress socks. If you are sitting, however, the bottom of your trousers may rise just enough to expose your dress socks for the world to see. This can either help or hurt your efforts to create an attractive formal outfit. If you wear ordinary white casual socks, it will bring down the aesthetics of your outfit while making it look more casual in the process. But if you wear stylish dress socks in an appropriate color, it will help you create a more cohesive, attractive formal outfit. A good rule of thumb is to choose dress socks in the same color as your trousers. If you're wearing navy trousers, for example, you should wear navy-colored socks. If you're wearing black trousers, you should wear black socks.
You Have to Wear a Tie
You don't always have to wear a tie with a formal suit outfit. Unless you've been invited to a formal event with a dress that specifically states "tie required," you can leave this accessory out of your outfit. Many men find neckties to be restrictive and uncomfortable to wear for long periods at a time. If this sounds familiar, another idea is to wear a bow tie, which won't constrict your neck like a necktie. As mentioned here, though, you can even go without a necktie.