Mixing the individual instruments that make up a song is typically reserved for audio engineers. The reason for this is that it takes a good understanding of how audio works both electronically, and how it works in your ears. That being said a simple explanation is in order to give a basic understanding of mixing instruments so that a song is well constructed

Drums: Typically Drums are spread across the stereo field from left to right with the Kick drum centered and cymbals spread evenly and toms flowing from left to right with the highest tom on the left and the lowest on the right.

Bass: Bass is almost always centered in the mix, the reason for this is that bass cannot be percieved very well as being decidedly "left" or "right" in the stereo field. bass frequencies are considered omnidirectional

Guitars: Acoustics often sit well a little to the right in the stereo field, and electric guitars are often panned to the left and right extremes to give a fat powerful sound

Keys: Keys are difficult.. keyboards generally have their own stereo signal and messing with it can be really tricky. My best suggestion is to leave them alone and just make sure they dont crowd the mix.

Vocals: Vocals need to be centered or just a little off center. a great idea is to have a flat vocal track that is centered and then pan an "effected" version either right, left or both to get a fatter fuller sound for the vocal track

Once everything has been placed in the stereo field the next step is EQ

Be sure to remove all the frequencies that you don't need by listening to the sound by itself and using a graphic equalizer cutting each frequency that does not affect the sound... once you have all the "empty" frequencies removed then you can sculpt the sound removing the frequencies that you don't care about... this is all about getting rid of as much sound information that we can so that the individual instruments don't "crowd" the frequencies. you can also look at a real time analyzer while you are adjusting things to give you an idea of how you are affecting the instrument.

Once each instrument has been manually EQ'ed and placed properly in the stereo field you should have a fairly decent mix. Adjust volumes and remix the instruments so that the song sounds like you think that it should.

when you have the song sounding like you think it should sound... you should apply a master EQ to the entire mix and some basic compression. At this point it is all about how you want the song to sound so play around and see what you like.

Remember this is a very simple explanation and glosses over a huge amount of critical information that can affect the sound of your song. There is truly no replacement for an experienced sound engineer, and hiring one can make the difference between a hit and a flop. Of course if you are just experimenting and learning then hopefully I have started you in the right direction.

Some quick tips.

• if you have an Apple computer, Garageband is a great tool to mix songs easily
• if you have a windows PC, Tracktion by Mackie is a great product with tons of plugins that are great including a mastering plugin that is pretty satisfactory for semi pro and even pro applications.
• when EQ' ing remember it is always better to cut a frequency than to boost it... boosting frequencies sometimes adds extra artifacts to the sound that can be undesireable
• Try to record your instrument at the highest volume that you can without causing "clipping" clipping is when the level meter shows red or goes over about -8 db
• I hope that basically answers your question here are some other resources to mixing and mastering information