For that price range, the Nikon SB-700 should serve you well! It is important that you move your flash away from the position of the built-in unit with that lens as I'm sure you've noticed that its long barrel causes a shadow to show up in photos when you use the flash. An external unit, mounted on the hotshoe or away from the camera avoids that problem.
Your D7000 also has a remote commander mode which allows it to trigger remote flashes, opening up all sorts of great possibilities. If you're feeling adventurous and want to save some money, a Yongnuo YN-468 II is a great option that costs a few hundred less than the Nikon SB-700 (just be sure that you're getting the Nikon TTL compatible version). The Nikon flash will be better documented and thus be easier to use, but the Yongnuo still produces very good light and the learning curve isn't too bad.
As for tips on how to use the external flash, guide numbers and whatnot, I think if you're an absolute beginner all of those terms may hinder more than help you since all that jargon can be very confusing. My personal experience was that I got the Yongnuo, slapped it on my D7000 (yes, we have the same camera) and started testing it on all sorts of subjects and situations. Yes, the first shots were not the best, but I learned more with those first shots using an external flash (particularly the ones where I triggered it wirelessly) than with most photography books I've ever read.
Bonus tip: the SB-700 and the YN-468 include built-in diffusers and rotating heads. Use the diffuser to soften the light (again, best way to learn is to take shots with and without the diffuser and see the result - just keep in mind that the diffuser does take range away from the flash) and bounce the light off surfaces to create more interesting illumination and remove the blast from your subject's eyes if shooting people or pets - trust me, they'll appreciate not having a flash blast directly go into their eyes.