You use lastly when you are saying what happens after everything else in a series of actions or events. They wash their hands, arms and faces, and lastly, they wash their feet. = finally. used when telling someone the last thing at the end of a list or a series of statements firstly
You use lastly when you want to make a final point, ask a final question, or mention a final item that is connected with the other ones you have already asked or mentioned. Lastly, I would like to ask about your future plans = finally