Deep breath, young grasshopper.......

About all your questions can be abswered with patience, and diligence.....and the search function.
Generally, the POSS test is given by utilities to prospective operator candidates. Search under POSS, AO, AOI for a start.
Job postings, and the requirements to apply for the job are in the top section of the home page - Jobs. You can setup a login ID for the Job Board, it is separate from your forum ID. Dust off your resume. Many of the postings don't require experience.
Browse around the site for a while. Look at some of the posts by some of the frequent names - get an idea where you can look for more information - the forum is setup so that you can go to a posters profile, and see recent posts. After you have read a million or so posts, you will start to get a feel for who and where to look for more info.
Do a search for the nuclear plants in your area - look at which utility owns/operates them, search the utility website for job postings.
Listen (read) until you start to know which questions to ask, and where to ask them. Make them specific.
Become a Goldmember.....

Good Luck, hope this helps.