Ok, have a social security number is your first step to receive your salary and start you financial live on the island. It could be a little be trickie as there is not information regarding what will be necessary once you arrive. I have discovery this in the hard way and will share my experience with the community.
Some facts you should know in advanced
1) You will only be able to apply for a Social Security Number after 30 days you entered as "worker" on the US territory. If you go before these 30 days you will loose your time, as your info will not be available on their system due to "slow" input from the immigration authorities (USCIS to be specific).
2) When you arrive in US territory and cross the Custom, make sure they enter the correct name on their system. If they not input your name on the same way it is written in your passport, you will not be able to have a Social Security Number, until they correct your name, which can take at least 60 days (if you discovered this after 30 days, you have to wait 90 days then!!!!). If the social security office tell you to go the immigration services to fix this (USCIS again), say them the procedure has changed and they need to fill out the form G-845 to start an agency-to-agency process, and they will forward a copy off all your documents (Passport, VISA, birth Certificate, etc) to Washington, and this one will take at least 60 days to get an answer and allow them to expedid you card.
3) They will require you an address to send the card by mail. They dont give the number there. If everything is ok, you should receive it in the address you just mentioned in about 2 weeks.
4) ... and noooo, you cannot track your mail by the internet or check how it is going the process anywhere else. Have faith and cheers they will call you if something goes wrong.
Now, that you have been aware, head to a Social Security Office with all your documents. There is one in Guaynabo, next to Plaza San Patricio - Ebano office Center. The office opens at 9h00 AM, but at 8h30 AM they allow you to get a code and wait for the interview. Try to arrive earlier, as there are many people going there every time and the waiting time is usually around 1h30 - 2h00. They close at 3 PM, except by wednesday which is 12 PM.
Hope this guide save you from many frustrations.