Your question is quite vague, more clarification would be useful. I will guess that maybe you are asking about benefits from your employer such as medical, dental, vision, 401k etc. If that is the case, the first hurdle is finding out if you qualify for such benefits. Qualifications are created by your employer within the boundaries of regulations surrounding each benefit. Common qualifications are age 21 and older, full time employee 40 hours a week.
If you qualify there is usually a waiting period, such as 30,60,90 days but for 401k can be as long as a year. Once your waiting period ends, you will be in a personal enrollment period for your benefits, after that you will have annual enrollment with the rest of the company...generally in the 4th quarter of the year.
There are also specific events in your life that will qualify you to make changes to your benefits; (Marriage/Divorce/Legal Separation, Birth/Adoption of a child, Death of Spouse/Child, Dependents loose coverage elsewhere due to employment status.
I hope this answers your question