Hey @ampersandrews! 
Welcome! Do you mean any Harley in general or just where to start in this current series?
Personally, I’m a big fan of Harleen, and both Black+White+Red series if you’re looking for something general. As for current/on-going/tie ins, like
I’ve enjoyed this one (and Poison Ivy’s solo series)–I know it’s technically sacrilege in Comicgeekland to not start at the beginning, but that’s often unrealistic. If you’re pressed for time, I’d start with the Dawn of DC stuff Harley Quinn #28 and go from there. Hope that helps, and let us know whatcha think if you go for it! 
Also, hello to everyone else! 
Hope all is well @d.Manha @Chain_Twix @chintzybeatnik @Cari_canto 