Good links from Gidge.
Keep in mind that the total sound of an instrument, especially with harmonics, usually covers a wide frequency range. You can't say that guitar "lives" at a certain frequency, for example. Guitars put out frequencies pretty much across the whole spectrum. Thus, you can't just cut a certain frequency in a stereo mix and lower the guitars in the mix, per se, as you'll be affecting content from other instruments as well. Sometimes that's what you want... if there's a big "bulge" somewhere in the overall recording you can tame it a little. Again, I'm talking about a stereo mix and not individual tracks in this case.
On individual tracks, I find it useful to "carve out" sections with some EQ to make room for everything in the mix. Low-end below 150Hz can sound big and full on guitars when they're solo'ed. But put that in the mix with everything else and you might find that the bass and kick have to fight through all that low frequency guitar stuff to be heard. Cutting some lows on guitars might sound thin when solo'ed, but in the mix you may discover that the bass and kick fill-in that low end. And not only that, they come through nice and clean 'cause theres room for them in the frequency spectrum. I often find that my solo'ed tracks use EQ settings I'd never think to use on the track by itself. But it works great in the mix.
It's also really really handy to train your ears to recognize what frequency is what. Those frequency charts are handy starting points, but every instument has it's own character. I've been slowly getting better with this, and keep working to improve. I want to reach the point where I can listen to a mix and just know that I want to cut the snare a little at, say, 1200Hz just by listening.
If you're completely new to EQ, you should learn the different types of EQ's and what they do. Fully parametric is your friend when mixing. You not only have good control over what frequency you boost or cut, but how wide that boost/cut is.