You can devote your life to learning about jewellery in general and diamonds in particular, but here are some rough guidelines. I'm sure others will chime in too.

Style - solitaire, a three stone or more?
Stones - diamonds, coloured gemstones, or both?

Other than the main stone(s), do you want some pave on it, eg diamonds on the shank / surprise stones?
Prongs or a bezel setting?
Set high or lower on the finger?

What ever the stone(s) make sure they are beautifully cut. David can advise better on this.

See what colour of diamond or other stone you prefer - yellow, pink, brown, blue, icy white or with a softer tint. Bear in mind that some gemstones are a little too soft for daily wear. I live in the UK and I see a lot of coloured stone engagement rings here, but they are usually sapphire, ruby and occasionally emerald. Softer stones like aquamarine need to be treated very gently.

See what inclusions you could live with - does it need to be flawless, or could you over look a flaw that wasn't too glaring or could be hidden under a prong?

My best advice would be to go to many, many jewellery stores and try on all the different styles, colours and sizes available. Then contact a trusted on-line vendor (and I would start right here at DBL) to put together a ring to balance your budget and preferences. For example, you could drop the colour and clarity (with a trusted vendor) to go up in size, or drop the size to go with a whiter diamond.

Hope that helps, but if I've misunderstood and your question was more specific, just shout out!

Jen

I think the important thing is not to "overthink" an engagement ring. So many try to make the ring so complicated. Many times lots of detail work is very costly and doesn't end up looking like it does in someone's head. So find something you like and work from there. Also, it's ok not to get what everyone else has.

I have a three stone engagement ring.