I want to become a tagahanga of that fanpop user, how will I do it?

Not sure if i understand you correctly but to become a tagahanga of someone you go to their bista sa tagiliran and underneath their name and paglalarawan you will find a little green tumawid labeled "Add to fanlist" click that and you're a fan. the user will receive a message telling them that you fanned them.
if you want to no longer be a tagahanga read this
link