A few of the formal parties specify dress code in the invitation cards. There are two kinds of dress code-black tie and white tie.
Black Tie: If your invitation card says black tie, then it indicates a semi formal occasion. Here you can dress your self in a cocktail dress or an evening gown. Since black tie is a semi formal occasion, you can choose the length and style of your gown as per the current trends. Strapless formal dresses are currently in vogue. Strapless dress highlights your face and the shoulders. Wear high heel stilettos with your evening gown. 
In case of a cocktail dress, do not wear a short length dress at the party. If you are taking your toddler for the party, select a dress from various infant formal dresses available. While selecting infant formal dress, you need to keep the climate in mind. The fabric and the design of the dress should be comfortable for your little one. Do not buy an oversized dress for your baby.
White Tie: White tie invitations are for formal occasions such as formal dinners, ballroom party, opera etc. If you are invited to a party with 'white tie' dress code, you need select a dress from elegant formal dresses. Remember, for white tie parties, it is mandatory to wear a full length dress. Moreover, your need to choose from modest formal dresses. Even strapless dress may not be acceptable at certain events. Ball gowns are generally white in color. Therefore, if you are attending a formal party, choose your dress carefully.
Accessories to Wear with Formal Dresses:
Shoes: Buy shoes that match with your dress. If you are attending a ball dance party, then wear heels that you feel comfortable while dancing.
Jewelry: Avoid chunky jewelry. The jewelry should match the gown. A string of pearls for white gown, or a small ruby or emerald set for red and green dress respectively will make you look elegant.
Shawls and Hand Gloves: These are considered mandatory for white tie parties. Shawls made in silk with embroidery would look good. You can try buying a Pashmina shawl. These shawls are hand spun have delicate embroidery on them.