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I have been doing research for over a year and have visited 50+ diamond stores/retailers and after price matching and haggling to the point of the salesperson wanting out of the situation I found that you well never get a deal from any standing retailer.
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If you are already looking for a ring. First you need to teach yourself the basics if you don't already know the 4 C`s cut, color, clarity and carot this is where you start. The site I use to actually purchase my diamonds also has information to teach you everything you need to know bluenile.com . After you know what you want in your diamond and what shape you are looking for you are ready to search for your diamond. 
This site is very easy to use after you select diamond in the search bar you should see show filters this allows you to select each aspect of the diamond you want 
After you have selected everything you want it should look something like this.
Then simply hit enter and search through tons of diamonds without insane markups. 
Your diamond will come with a certificate I recommend GIA you will find this information in your diamonds details. From this point you can continue using the site to build your ring or have it sent to you and have a jeweler build it. I used the site.
Depending on what state or country your in the probably don't charge taxes and they also offer financing. I spent 3,000 on my ring and it got appraised for 9,000 and I tried to buy it at a diamond retailer and the salesperson laughed and asked if I had 20,000 to spend.
Over all you will be very happy with your purchase and you will get more than you expected.