heres the wiki for freeing up rootfs: http://wiki.maemo.org/Free_up_rootfs_space

dont be disheartened it can be confusing at first but you'll pick it up.
if you're worried about what the previous user has done to it you may want to consider doing a full flash and restore the operating system to factory standard. okay that also sounds daunting but there are step by step guides on here that will get you through it.