The first thing to do is to look at your hands objectively.
The general rule is that if you have:
Short fingers – an elongated diamond such as a marquise or oval can make short fingers look longer (but beware of choosing too big a stone, as it can have the opposite effect). Wide bands can tend to make fingers look shorter than they are, so try and choose a band that is in proportion with your fingers
Long fingers – can easily wear bold ring styles.
Large hands – keep away from delicate styles as they tend to over-emphasize the size of your hands and make the ring look smaller.
Small hands – delicate designs can draw attention to the dainty nature of your hands