You could always do a Party Finder group - if you explicitly say lower dungeons and tanking practice there'll no doubt be some nice folks who will happily help. Otherwise, as previously mentioned - Duty Support is a good one for learning the basics with no other players there, ask your FC is they would mind helping, or just queue for a specific set of lower dungeons (granted you don't get the roulette bonus but that's the gamble of roulette - one day you could be in Sastasha, the next you're in <insert current hardest/painful dungeon>, but at least you get some sweet exp bonus!).