I am moving to the states next month. I have my I-551 visa which I guess becomes my green card once I enter the states. But I wanted to ask how long and what I need to do to get my Social Security number? Will it just get sent to me? Or do I have to apply once I get to the states.

I am just trying to figure out how long it will take before I can start working once I get out there

If you are age 18 or older and checked off of the DS-230 that you wanted to be assigned an SSN and have an immigrant visa, you should receive your SSN card within 3 weeks of entering the U.S. If you don't receive the card you need to go into an SSA office.

RM 00202.315 Enumeration At Entry
http://policy.ssa.gov/poms.nsf/lnx/0100202315

Take proof of age, identity and work authorized alien status.

Birth certificate, passport and I-551 card or MRIV with temporary I-551 language should be enough to take into the SSA office.

If you have an MRIV with temporary I-551 language you can start working right away, but might hard to find someone that will hire you without an SSN.