bongo is right; there's no magic frequency to turn up for each instrument; but there are parts of the spectrum in which particular qualities of instruments live.
If, for example, you want the mix to feature a singing clarity in the bass guitar's high mids, you'll make sure that the bass is prominent at ~ 800 Hz. But rather than CRANKING the bass track at ~800 Hz, typically you look to the arrangement to make sure no other instrument with strong 800 Hz content is covering the bass part.
Here's a good starting point for thinking about how instrument voice qualities relate to frequency ranges.
If there is unwanted frequency masking between instruments, decide which voice should be prominent in a particular range. You can reduce frequency masking w/ eq somewhat by *dipping* the offending instrument at 800 Hz. But generally the better solution is to fix the arrangement, so that the voices leave one another space--in frequency and/or in time. (And of course, sometimes you want instruments to blur together in a particular range—that's why it's called
mixing.)
As the easiest to manage first step with eq, experiment with putting a parametric eq's high-pass filter on all the voices in a mix except the kick drum and bass. A high pass filter, or HPF, will allow all frequencies *above* its set point to pass. Most channel voices don't contribute much to the mix below 100-120 Hz, so you can usually clear up the mix's low end—that is, give the kick and bass room to breathe—by high passing everything else.
Play with setting the HPF for a particular voice. How high can you bring it before it cuts into the essential quality of that channel's voice? Find that point, then ease back the HPF a bit until the instrument voice has its appropriate bottom again. Move to the next channel. Rinse. Repeat.
You'll notice that for some instrument voices, the bottom floor is a lot higher than for other voices. You'll also notice that for any particular instrument, the mix-appropriate setting for a HPF depends on the song. For studio work, you might change the HPF and other eq settings from bridge to chorus, or within individual lines. But for live work, you'll tend to set HPFs and leave them.
Once you get a handle on HPFs, you'll notice that the same principle applies to low pass filters. Most voices have a frequency above which they're not contributing anything useful to the mix, but only masking the voices that live in that range. (For guitar amps in a rock band, for example, I'll *usually* have an HPF set no lower than 100-120 Hz, and a LPF set no higher than 6 KHz.)
Now, between setting the HPFs & LPFs, you've worked out the characteristic range for each voice. Within *that* useful range, eq adjustments will change the character of the voice. Sometimes, you'll want a small boost to bring out some part of that character. More frequently, you'll cut eq in part of an instrument's useful range to create space in the mix for some other instrument voice to speak.
