The main rules of visiting the Chernobyl Exclusion Zone are the following:
visits are forbidden for the persons younger than 18
visits are only allowed within specially organized groups with officially issued program of visit and accompanied by a qualified person
to proceed through the checkpoint you will need your passport
it is forbidden to visit the Zone under the influence of alcohol or drugs
your clothes should cover most of your body
You can read more about the rules of visits on Safety in the trips page