What dirkr said. There's far too many potential issues for anyone to give you definitive answers, and it will also rely heavily on where you are based, whether you sell stuff for other people, and who you sell to. There may be things like VAT, sales tax and withholding taxes that might come in to play... and depending on your location, the rules are going to vary considerably when it comes to you paying taxes. You might be able to register as self-employed, you might have to register as a business, if you're in most European countries you'll have to register for VAT, charge it and then remit it to the correct department etc etc.
Nothing wrong with asking for a bit of advice here, but you should just take the answers as opinions and anecdotal evidence to give you a rough idea of what may or may not be involved... avoid taking them as gospel.