Help - social security numbers/medical cover/tax (Paris)

Hi - I moved to Paris to work 2 months ago and the people I work with have given me a variety of differing info on how to go about getting a social security number and medical cover/signing up with a doctor. I will not have a permanent address until Jan 5th but can anyone help in explaining the processes and what documentation I need to have?

The other thing I am unsure of is the tax situation. I'm English and we usually have a maximum of around 3 or 4 deductible items on our salary slips. Just received my first one with my new employer and the list is as long as my arm!!!! I seem to have paid out around 30% of my salary on various things but I have been told it doesnt even include tax. It has been suggested I put aside at least another 20% to pay the tax later. If tax is not included in the salary, whatever am I paying for? and if I do indeed have to find money to pay tax as well, how do I find out how much I will have to pay? It seems an awful lot of my salary to be paying out.

Many thanks
Tina