How can you join the Sea Org?

It will take a billion-year-long commitment. If you sign the billion year contract, they won't make it so that you will live a billion years, however. Instead, you will live a normal human lifetime at most, but will be expected to return to the job after you are "reincarnated" in your next lifetime. Again, there is no guarantee or evidence that will happen, other than your belief, also known as faith. If you sign the contract, that means you have that belief.

In this lifetime (and all the next ones if they happen), you will have to work very hard for very little pay, and live in cramped dormitories without luxury. Privacy is not part of that. Strict obedience is mandatory, and higher-ups will control where and how you work. You may be permanently separated from your spouse, if that is deemed necessary. This is all stuff you should make your peace with BEFORE you sign the contract.