First, what type of metal will your partner love? There are generally three options for metal color--yellow gold, rose gold, and white gold. In many cases, a white gold band (or shank) can be interchanged with platinum for durability, preference, or allergy reasons. Some engagement rings incorporate more than one color metal for a more unique look.
Next, you'll have to pick a style of setting. We've included images of the various choices you'll have.
Lastly, you'll choose a diamond for the center stone of the ring. If you're purchasing a halo ring, you'll likely be limited to the desired shape and weight that the mounting accommodates. If there's no halo wrapping around the mounting for the center stone, you have much more flexibility when it comes to diamond cut and weight.
Does your partner LOVE the unique diamond shapes such as the marquise or pear? Your jeweler will be able to help you choose and set any of those shaped diamonds into your ring.
When it comes to choosing an engagement ring, you're making an important decision that will be relevant for the rest of your life. Take into consideration all of the hints and suggestions you've received from your partner or otherwise uncovered from her friends and family. She's going to love what you choose.