North Girl
Becoming a witness is a process that involves several steps. First, somehow you come in contact with them and display an interest in their religion. After being called on for a while, you will be offered a "Bible study" which consists of going through one or more publications with the witnesses Then you will be encouraged to begin attending meetings. Assuming you have gotten to this stage, the next step will be the field service, although you must receive "approval" to be permitted to do this (if you have a basic understanding of JW beliefs, and if you are not living a lifestyle they disapprove of- i.e. smoking, living with someone, etc, you will be approved).
Its a steady progression, and if any point you balk, they will try very hard to get you to continue. Once you have started attending all the meetings and are participating in field service, the next logical step is baptism. And it will be stressed to you how vital this step is if you are to truly be part of God's organizaition and have the wunnerful hope of living on paradise erf with the rest of the JWs.
As long as you are not baptised, you can make mistakes, and as long as you act repentant and make it clear that you really really want to be a JW someday, you will be "forgiven." Its all about getting you in, and getting baptised is what will seal your fate.
No Apologies of the long-winded but hopefully useful answer class)