Which screen reader would be best to test site accessibility and how to configure that?

I will preface this answer by stating I’m a totally blind individual who uses Jaws as there only screen reader. I've played around with NVDA as well but have l9imited experience with it. Jaws is the most widely used screen reader at least in the US. If you can only use one screen reader I would pick it with the default settings. Both Internet Explorer and Firefox work with Jaws and both are widely used. Another screen reader you could use to test accessibility is NVDA this is an open source screen reader that works well with Firefox but not internet explorer. I would say if cost is an issue use NVDA with the latest version of Firefox, and if your site is accessible using that setup it will most likely work with Jaws. For a complete list of screen reading software see this

Installing and starting a screen reader isn't enough to do good accessibility testing. You won't know how accessible your site is until you turn off your monitor and unplug your mouse. Getting good enough at using any screen reader to do that will take time. The only sighted people I know that are efficient screen reader users either work for the screen reader companies, or do assistive technology training as their job. So while you can use a screen reader to test your site's accessibility the learning curve for a realistic test is quite high.

To answer your question directly, I would use JAWS with default settings in your target browser. If you can only afford one license, then use NVDA or Chromevox for your developers and give your Accessibility expert the copy of JAWS.

Keep in mind that while making sure your site works perfectly with a screenreader is very important, this only helps the blind. There are many other types of disabilities (e.g., hearing, motor, and cognitive disabilities) and to truly be accessible, your site needs to support those users too.

WCAG 2.0 is the best standard for making your site accessible to as many people as possible. There is A LOT of WCAG 2.0 documentation though, so I would start at webaim.org, http://webaim.org/standards/wcag/checklist if you are new to it, but do use the real thing http://www.w3.org/TR/WCAG20/ when you are ready.

Also, keep in mind that even if it "works" with a screenreader, it may be annoying (blind users rarely read top-to bottom, make sure you put in solid structure with headings and ARIA lankmarks) or it may not be giving a blind user the same amount of information that a sighted user might get. For example, helper text next to text inputs will be missed by a user tabbing through a form (fixes: hide a copy in the label with CSS, make the helper text the actual label, or use ARIA-describedby) - a good way to make sure it more than just "works" is to have your JAWS tester not be familiar with the site.