Choose your gear. You need two turntables or CD players in order to have two audio sources, and a mixer, which will allow you to blend the two audio streams. When choosing between a turntable and CD player, keep in mind that you'll also need to have your music in the corresponding media, LP or CD.
Get to know your equipment and familiarize yourself with music basics. A common misconception is that a DJ simply plays tracks randomly, which is not true. You'll need to know how to find tracks that are similar and how to adjust the beats per minute (BPM) of two tracks to get to an identical value which will allow you to mix them together.
Practice. There are no "tips and tricks" to learn faster. Once you know what you have to do, you'll have to practice that until you get a grip on it. Don't start with more complex mixing techniques such as scratching before you master the basics.
Improvise. Music is all about fun and having a good time, so have a good time while playing with it. Don't be afraid to try something new.
Be flexible. When mixing at a party, don't stick to your home-made playlist, especially if you see that it keeps people away from the dance floor. Learn to "read" your audience by trying several types of music and sticking to the one that has them up and dancing.