A good set of DJ turntables has direct-drive power and adjustable speed. Direct-drive tables won't slow or lag when dragged back or forward; adjustable speed allows you to blend different rhythms together. Use a mixer that is the right size and width to fit between your turntables and give you easy access to the controls.
Listen to the record playing on the left-hand turntable in your headphones. Keep one headphone off of your ear so that you can hear the mix. With the fader all the way to the left, start to play another record on the right-hand turntable. Listen to the two beats and adjust the right-hand turntable's speed until the beats of the two records match. Restart the right-hand record at the appropriate time and blend the two tracks together by sliding the fader across.
If you want to scratch records and perform other tricks, you will need specialized record needle cartridges. Avoid long, thin models; look for cartridges that are somewhat heavy and bulky. This type of needle will have a better hold on the record's grooves.
Tape a penny or nickel to the top of your needles to keep records from skipping when you drag them back and forth. You can also cut a circular piece of wax paper to the size of the turntable and place it beneath your turntable pad.