The most important thing to do when buying a ring is to find a great jeweler, one way to do this is by asking family and friends who are recently engaged where they went and how their experience was.
Next you need to set a budget so you don't spend too much, it is suggested that men spend 2-3 months salary on a ring.
Third is knowing the lingo, the C's. While many people think there are only 4 C's when picking an engagement ring, there are really 5. They are Carat, Color, Clarity, Cut and Cost.
Carat: This is the weight of the diamond not the size.
Color: This is self explanatory, the color of the diamond is rated on a scale starting with D as colorless.
Cost: This is the most important of the C's because this will determine what type of carat and clarity you can afford.
Lastly: Remember the person who will be receiving the ring, she will be wearing it forever and it is important to consider her tastes not just your own or the tastes of the people you take with you.