Income Tax

I work on a contract basis for a number of small companies and am paid nett of tax. I have been told that I need to complete a tax return but I don't know how to go about it. I though that as I was paying tax anyway it would all take care of itself but not so! So, if anyone could tell me what the process is for tax returns (where so I have to go?  When do I have to file the return? etc) I would be really grateful. Someone suggested that I register for partita IVA and that that my be simpler. I don't know! Many thanks