How can i use audacity as a dj thingy with cross fade and playlists
Audacity does not do “play lists”.
Audacity does not do real time effects.
You can cross fade one track with another by importing the 2 tracks, line them up along the time line, then use the Fade Out and Fade In effects.
There is a comprehensive Help manual (see “Help” menu)
Audacity is a post production tool. Capture the work, music, or singing in the computer and and mix and work on it sometime next week for example. Audacity only does two things in real time, capture and playback. No effects.
Koz
You could try iTunes - it’s really a juke-box “thingy” rather than a DJ “thingy” 
but it does have playlists and you can set the crossfades. And best of all, just like Audacity, it’s available for free 
Apple work on the basis that you’re likely to buy an iPod if you get iTunes and build your library in it - and thus provide the software FOC. But for DJ use you can just run iTunes on a laptop or Macbook “thingy” - the interface on those is better (for your purposes) than the reduced iPod interface.
WC
This is a brilliant free DJ thingy: http://www.mixxx.org/
(Windows Mac and Linux)