I Need Advice On Becoming An 'Autonomo' in Spain. (Barcelona)

Hi All,
First of all, Feliz día de los reyes magos.

I am thinking of becoming an 'autonomo'. I I have read up on information on the web but the info is not so clear.

I have a couple of questions and any advice would be greatly appreciated.

1. If I want a take home income of Protected content per day nett, how much should I actually be billing the company that engages me?

2. How often do I have to submit my accounts? Do I need to engage someone to do it for me (If yes, any recommendations) or is there any website that can guide me to do it myself?

3. How much do I need to contribute to my social security and my pension?

I have more questions but these are the most pressing ones.

Cheers,
Rey