Jon McCammond maintains a list of conferences in geometric group theory here.
Jesse Johnson maintains a list of conferences in low-dimensional topology here.
I hear about many conferences from the geometry listserv here.
There are also interesting conferences (slanted towards algebraic topology, but with a broad focus) on the list here.
As far as your question of how I decide which conferences to attend, at this point in my career (on the tenure track, but not yet tenured; I started this policy when I was a postdoc) I mostly only go to conferences where I am speaking. Sometimes I'll attend a conference where I'm not speaking if the conference has an unusually strong lineup of speakers, but I already travel far too much.
When I was a graduate student, I basically went to conferences suggested by my advisor.
