And I would be pleased about a less childish reply.
Jan, I do apologize for having been so rude, the less I can say is that my answer wasn't really brilliant...
(I think that when i replied i was in a bad mood and the word childish comes to my mind remembering an awfull avatar, see which one i mean ?)
I will rephrase what i meant in a more productive way :
(1) Basically, an "addon" is a folder (compressed in a ".pbo" file with pbo tools, lot's of them being available here and there, i'm using PboX), with a config file (text file with .cpp extension) and a 3d model (.p3d extension, made with O2 light software), with textures (.paa, .pac or .jpeg files) ;
(2) What kind of addon do you precisely intend to do ?
(3) The best way of learning is to search through OFP/ARMA forums in the "how to" topics, and to use some very good tutorials like Brsseb one that you can find here :
http://ofp.gamepark.cz/_hosted/brsseb/index.html