I'm soon going to buy a new 27 inch iMac with the improved display. I'm a fairly novice user of Lightroom and Photoshop. How do I take advantage of the p3 color space that the iMac supports?
I'd suggest you dive into the documentation for Ps and Lr. They both can make use of the AdobeRGB gamut, and there are pros and cons to that, especially depending on your output medium. Even with the new monitor you'll probably also need to take a look at colorimeters to calibrate your display. I've found this site has some really great educational material on color (or colour, which is a wider gamut unique to GB apparently): http://www.cambridgeincolour.com