Your Social Security card should be mailed to you within 2-3 weeks. In the meantime your employer's Human Resources office will probably want to see your receipt showing that you have applied for a card.
After showing your Social Security Card to your employer put it away in a safe place.The Social Security Administration advises that you should not carry your card with you.
Protect Yourself Against Identity Theft
Social Security numbers are often used in cases of identity theft. Be cautious in sharing the number with anyone, and keep the card in a safe place with other important documents, rather than carrying it with you and risking losing it or having it stolen.
For more information, please see the Social Security website page on Identity Theft.
Replacement of a Lost Social Security Card
If you lose your Social Security card you will need to follow the same procedure to apply for a replacement card, which will have the same number. Note that you will again need to show current proof of employment and proof of work eligibility.