I haven't paid social security in Shanghai before, but now the company wants to pay it, but I don't know where to start if I want to transfer it before.

If the company you are looking for also pays social security, you can open a social security transfer letter of introduction and social security payment certificate at the social security bureau where your previous company paid social security, and you can continue to pay it when you get the new company.

If you are not looking for a job now or the company you are looking for has not paid social security, you can pay it at the human resources company, and it is the same. You have to open a social security transfer letter and social security payment certificate from the social security bureau that paid the social security before, and get it to the human resources company to continue to pay.

Pay social security yourself. As long as the transfer or later merger is completed, the social security paid by the company before is useful.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.