How to go into the attic/roof area of a home safely?

I hear a rat in my roof because every night I hear the critter running around. I've decided to try to kill it myself by laying traps in the roof at different areas. However, I have never been inside my roof before. Is it safe? Is it like a floor can I walk anywhere? Can there be mold? Do i need to wear a gas mask if there is mold and would that type of mask work? Please inform me of how I can get up there safely and any precautions I need to take. Thank you