Good Question. I'd also like to know, why did Jesus say that in order for any man to see the Kingdom of God, he must be born again. Doesn't this only apply to the 144,000 ? What about the rest?
I believe that the 144,000 is a symbolic number for the jews. The great crowd is the gentiles (the rest of God's children). Both the 144,000 and the great crowd will be in heaven. (Rev 19:1)
Now on to being saved:
We must believe in Jesus to be saved. Period.
But what does "believe" mean?
It can't be just knowledge that he existed because even the demons believe and yet they shutter. (That's in James 2:19)
It must be something a bit more -- but yet not something you work for because salvation is a free gift. Eph 2: 8,9 For by grace you have been saved through faith; and that not of yourselves, it is the gift of God; 9 not as a result of works, so that no one may boast.
So, what is it? The saving faith means putting your trust in Jesus alone for salvation.
If you are saved you will: Repent (means when you do wrong, you will ask for forgiveness and make a U turn -- not meaning that you will never mess up -- You will!!)
Receive Jesus as the Lord over your life (Meaning you will make decisions based on what you think he would want you to do and not what you think is best)
So, you don't actually "work" for it like field service or meeting attendance. Its just a change in relying on God in prayer instead of yourself all of the time.