Hi dabeer,
In what part of Spain will you be working? Will you be working as an English teacher?
If you speak/read Spanish, I recommend the following websites.
Ventanilla ÚnicaCómo Hacerse AutónomoHacerse AutónomoIf you don't control Spanish well enough yet, let me know and I will translate the basic information for you.
As far as my experiences are concerned, it was probably one of the best decisions I have ever made and have not regretted it since. Working autónoma allows me more freedom and independence. I control my hours, my rates and my earning potential is unlimited...limited only to the time that I want to dedicate to teaching.
Let us know how it works for you!
Good luck!
R-