Hey guys,
I'm trying to start up a graphic t-shirt line. I've come up with several designs and want to incorporate wording. I don't know anything about the "font business" and could really use some help.

Can I just download and use a font for a shirt or do I have to get permission/license?
Can I use a font for branding purposes?
Any tips when using downloaded font on sellable merchandise?
Anything I should know on the legal front?