This is a brief guide to help you begin your quest as a "good" DJ, meaning you'll have the ability to match the tempo of the songs you want to mix together, and thus mix songs together seamlessly.

The first step in learning this techinque is to get two records (or CDs) that have a clearly defined percussion section.  They don't have to be the same song or artist, but it will help you to get started if you can hear the drum beats clearly over any other instrument or vocal in the song; it's the drums that provide the most cosistent tempo, and therefore determine the number of beats per minute (BPM). 

Once you've made your choices, start one of the songs on your playback device.  Now, use headphones to "cue" the second track, meaning that you can hear the second song in your headphones, but it's not coming through on the main speakers/monitors.  If using a turntable, find a specific part of the drum track that you can hear clearly and move the record back and forth over that point in time with the song that is already playing -- this will most likely be the bass/kick drum, which usually occurs on the "one" and "three" of the downbeat ('one, two, three, four, etc.), or the snare drum, which is usually on the "two" and "four" of the downbeat. 

When I first started out, I used the bass kick as the reference point, since it was fairly easy to determine the "one" of the four-part loop.  However, I learned to use the snare drum more often, for a couple of different reasons.  First, the sound of the snare drum is (usually) very clear and stands out against all other parts of the drum sound -- this is very useful in a loud environment (a nightclub, for example).  Second, the snare drum is usually the most consistent part of the drum kit -- the bass drum is usually varied and doesn't always occur at the same part for every 4 counts, especially with newer electronic and hip hop music.

Once you've picked the specific drum sound that you want to use as a cue point, count with the original song's tempo, and then release the record (or hit 'play' on the CD) and listen for how the two songs "drift" from each other.  If you released the song at the right point, you should hear the two identical drum parts at exactly the same time.  If their BPM's aren't matched, you will hear one of the drum parts before the other identical part.  If it's the song you're mixing into, then the tempo of the new song is too fast; if it seems to happen after the original song, then the tempo is too slow.  Make a slight adjustment to the pitch control of your playback device, find the cue point you used earlier (I like to use the snare drum on the "two" count), and try this process again. 

It may take you several minutes (or longer) on your first attempts, but with a few weeks of practice you'll be able to match the two tempos in about 30 seconds.  When first learning this technique, it really helps if you have a consistent beat for your drum sounds; electronic music is the best for this, as a live drummer (soul & funk) usually drifts a tiny bit on every count-through. 

Hopefully this guide will get you on your way to making great mixes!