Starting steps

Hi, suppose I have a piece of software which I'd like
to sell over the internet. What are the necessary steps
to make it legally? Registering a domain, etc. is out
of the question, but I'm interested about the legal issues
(that is taxes among other things) that I should be aware of.
I see many small companies such as "xxx Corp.", or "Inc."...
and I'd like to know if I have to register the soft, a brand, etc.
BTW I'm living in Spain, but I suppose that other countries
experiences will help me.

TIA.