like n0c said, they should all be fine.
differences to note:
debian has nice package management system called apt, that allows you to get software from remote ftps very easily.
slackware has no real package management (.tgz arent used by anyone) but has nicer init scripts than debian IMO
freebsd has ports which is like apt in many respect and is very useful.
freebsd is obviously not linux, but if youre using bash, most of the commands should be identical. i started on slackware and i like being forced to do things manually at first. but then i migrated my main workstation to debian since it has nice packages. ive been meaning to learn more about the bsd system as i have very limited experience.
few tips, when u start, give yourself a project with a specific goal. when istarted, i want to build an ip masquerading box for my cable modem. then web server, then dns, and so forth until u understand the system. dont be afraid to mess up. each failure is a learning experience blah blah blah.
enjoy and remember, when u get frustrated from not knowing something, you love the technology and you love learning new systems.