how do I refresh my iTunes library?

I think I need to refresh my iTunes library. There are a few reasons.

First is from when I was using SimplifyMedia. For some reason I could never get it to "see" all of my songs, so I stopped using it.

Second is my album artwork. For some reason, the only time I am able to get album artwork these days is if I download a song directly from iTunes. Whenever I try to import songs from a new cd, even something current and well-known, I get no album artwork, which is annoying. The track info is found but no album cover. It led me to getting TuneUp, which is a third party program that finds missing artwork. TuneUp does an okay job but some of the album covers it finds are either wrong or just strange. I would prefer to fix whatever is preventing iTunes from giving me the correct album covers and I'm wondering if it has something to do with perhaps a faulty iTunes library file.

Third is my iTunes library used to be a Windows file but I've been a macuser for years now. See, I used to be a PC user and used iTunes for Windows. I then got a MacBook Pro and had both a PC and a Mac at the same time for awhile. I didn't like that all my music was just on my PC, so I experimented with using an external drive that was compatible with both a PC and a Mac and stored my iTunes music folder on it so I could access my music from both machines. Then at some point I went totally Mac and gave up the PC. But I had simply copied my iTunes library which was created by Windows to my Mac and was able to just use it. But I've always wished it was a purely mac file. And at this point all of my music is now on my laptop - I no longer use the external drive.

So I was thinking I should totally refresh my iTunes library. What's the best way to go about doing so? I won't mind if I have to reimport all of my mp3s, then consolidate and delete the extra copy of my harddrive. But could someone please outline the proper steps so I don't screw it up? Thanks!