Determine what kind of people will be at the party and prepare a list of appropriate tracks. If you know that the people attending will primarily be looking to dance, include some upbeat dance tracks.
Select the tracks that you would like to play. Ideally, you want these tracks to flow together seamlessly, with no breaks between songs. To do this, determine the key and tempo of each song. You generally want to find songs with similar tempos so the transitions won't be abrupt.
In order to make your mix sound continuous, you need to purchase a DJ mixer. A DJ mixer comes equipped with a crossfader so that switching from one track to another is a smooth process.
Use the equalizer knobs on the DJ mixer to control the frequencies of the tracks you are mixing together. The equalizer knobs are typically labeled "bass," "mids" and "treble." The bass knob controls the low frequencies, the mids knob controls the midrange frequencies, and the treble knob controls the high frequencies.
DJs often will play two tracks at the same time to create a mashup track. If you are playing two tracks at the same time to create a mashup, be sure to turn down conflicting frequencies; otherwise your mix will sound muddy. In a party environment, the bass is generally boosted in order to give the mix more kick.
Use headphones to preview how your mix will sound. Use the headphone cue button on your DJ mixer to play a track through your headphones. Tweak the track by turning the equalizer knobs on your DJ mixer so that the frequencies of the other track do not conflict with it.
Always practice mixing songs at home before you mix in front of a crowd. Using the DJ mixer is like playing an instrument--they both take practice to become good at the task.