For finding accessible places, one really just needs to do research to find one or just randomly stumble across a ruined building. One can even ask around the local place to find if there are any such buildings around.
For precautions, there are a plenty. One should tell someone where they are going in case there is an emergency. Bringing a flashlight, extra batteries, gloves, and water and some food (never know how long one would be exploring but one at the very least should bring some water). Also covering clothes and nothing baggy that could get caught on things. A mask/respirator would also be handy.
Then there’s the whole illegal issue as well. Some places do not care if someone does urban exploration in buildings while others care. Best to find out which it is. Then there are places where it is illegal to explore, but finding the owner and asking permission can get you inside but one would have to follow their rules once inside the building.
A site to get some more information from would be Urban Explorers. It can provide more information than I can since I have no actually done any exploring that was not open to the public and the ones that are, are not completely abandoned.