You can apply for a Social Security Number at https://www.ssa.gov/number-card however, it may be difficult depending upon where your case stands in the immigration process.  The current Immigration forms contain a section to apply for a SSN when applying for Immigration Status, and is usually the easiest and fastest way to obtain a Social Security Card for the first time.  See https://www.ssa.gov/number-card for a replacement card if necessary.