Hi, and welcome to the forum.

Our resident Spanish moderator is away for a few days, but will no doubt be back to answer your question in more detail.

If you look through some of the old messages here, you'll find some information. Evidently setting up a small business in Spain is similar to what it is here in France. You have to formally register your business, which is a paperwork blizzard. Then, there is a fixed "contribution" for national insurances that you will have to pay each month, whether or not you have income or customers.

But hang on a couple of days and there should be some other folks through here with more information for you.
Cheers,
Bev