As Starfish mentioned make sure you know what the prof does, look up their research profile online or in simple terms start stalking
. It's also good to look into what publications they have written so you can throw that into the conversation which shows you did your homework and have interest in the prof's work.
I would make sure to take any documents you might have such as your resume/CV or sometimes reference letters (copies of them) it just gives you an overall edge and shows you are serious even if it is not a paid position.
And of course as mentioned start building your thoughts on why you want to volunteer, what's your goal long term with this volunteer experience, what can you offer to the lab that other students can't?
And good luck!