Welcome in the bitcoin forum
I can only buy dogecoins from some other user of dogecoins.
You can use exchanges sites like cryptsy or coinedup, if you don't mind them being a bit slow... Anyway, of course all coins come from other users. Every bar of gold comes from someone who mined it after all, same here.
Does everyone set his own currency?
Uh?
Where did he get the coins.
From how every coin is generated in a cryptocurrency: via mining. There is no a "central authority" giving coins lol
What is mining and is it possible with a crappy laptop?
It is how the whole thing is secured, and also create the new coins. With your laptop? Not exactly a good idea, you will overheat it and make like no profit...