How to set the default to unfolded when you open a file?

set foldlevel=99

should open all folds, regardless of method used for folding. With foldlevel=0 all folded, foldlevel=1 only somes, ... higher numbers will close fewer folds.

You can put this in your .vimrc: au BufRead * normal zR

It declares an automatic command (au), triggered when a buffer is read (BufRead), matching all files (*) and executes the zR (opens all folds) command in normal mode.