I've been djing and promoting shows for about 7-8 years now. The idea of DJing is to play music for people. Whether to give them something to talk over, or dance too, or whatever.
First things first is you have to find your audience. Is it your friends? do you want to play house parties? clubs? shows? If casual (as you stated) then your probably just going to end up doing your friends parties. I've done tons of house parties, and they are a blast.
doing something like that entails music, equipment to play (turntables, cdjays, abelton live software), mixers, speakers, amps, maybe some lights (if you want to get fancy).
Also are you going to play the whole night? then either a mix list to just hit play (which isn't really djing) or get some djs to pad the night.
Then you have to figure out what kind of music you want to play. I personally am a Drum and Bass dj (mostly) which is a faster type of music. This comes back to knowing your audience. Are they their to rock out to some metal, or grind to some hiphop? there is tons of music to play and finding the stuff that you love is key. If you love the music you play, people will get into it through you (unless they hate it, which you'll always run into)
I do a Q and A on djing on Guy.com actually (which seems to be having some Database errors right now, so no link, just go and check the forum discussions under 'ask a dj') which will have a bunch of info to help you figure out what you want to do.
Also I'd search for some DJing mixes, to hear how music is put together. ( i have a bunch at http://www.djcubed.com if you like bass heavy electronica)
i hope that helps, :)