Mixing is basically the art of making everything sound good together. You're mixing 10 - 50 tracks depending on your song to all sound good on 1 master track.
EQ basically is used to cut out frequencies of each sound so that it has its own 'space' in the frequency spectrum. for instance, Kick normally hits at 80 - 130 hz and also has a spot at the top at around 1000hz to get the air of the kick and give more presence. Now if you have a snare or a horn sound maybe that hits in the same frequency it MAY sound better to cut some of those frequencies out with a high pass filter or use a high shelf filter so that the kick drum sits on top of those sounds. EQ everything for the most part. How you EQ each sound is the art of it.
Next big piece that ties together with EQ is panning to give sounds their 'space'. Really only the Kick, Snare, Bass, And vocal are supposed to be in the center. That way your ear focuses on them. Also makes it so you can really hear those elements and the other sounds at the same time. A good way to hear this is to take a drum pattern of just a kick, snare, and high hat - then pan the high hat just a little to the left and pay attention to how it makes the snare sound like it has slightly more presence. The more you get into lower frequencies the more you dont want to pan. below 200hz is a 'standard' but sometimes it may sound good to not follow that one.
A lot of people have mentioned compression and a lot of people also dont understand it. Compression is awesome. What it does can be confusing but once you get it, you get it. When you look at a wave there is a high point and a low point. What compression REALLY ends up doing is levels out the high points and brings the lower points up with what is called 'make up gain'. Also the attack knob is used to give a sound a really nice transient(the first couple seconds of the sound are unaffected but still have the make up gain so its a nice hit. these make your ear 'hear' the sound as being a little louder. This comes to mind with drums a lot.) I'd probably recommend searching youtube for compression videos. Young Guru has a nice video about using compression on there.
As far as sampling goes, everyone has their own way of doing it. My personal taste is to EQ and cut the low to between 200 - 600hz depending on the sample. Sometimes maybe higher. That way You get rid of the bass and drums in the mix. Also it may sound good to remove some of the high end to get rid of the high hat or symbols in sample but its different for each sample. That could make it sound bad sometimes. at that point, depending on the sample I normally will do some stereo widening to the sample so that a vocal will fit nice in the middle.
Thats the start to mixing. There is a LOT more to get into like delays, reverbs, parallel compression, stereo widening, but that will get you started. Youtube search those terms once you feel you have a solid understanding of EQ, Panning, and Comp. Mixing honestly my favorite part about producing. It is an art form just in itself.