Commit on sourceTree on new branch without commit on old branch

[Amit Cool]

Dear Team,

if we forgot to create a new branch for new functionality and developed new functionality without commit on old branch then Could you please guide me if we have any procedure to commit on new branch with source tree because it is not allowing me to checkout with new branch without commit with old branch 

Could you please guide me how can we use stash ,Discard and Tag Button as better way in source tree terminal window and what is better user for all same button (Stash, Discard and Tag)

Thank You 

Amit Gupta