Resolving Git Merge Conflict

Question

Working in git with limited 8-10 years experience using CVS needing to understand what git repo is doing.

I created a branch off master with changes that I committed and then pushed back to master. Now in the same project I created another branch with a different change to one of the same files. As I go to create a Pull Request for my latest branch B it shows commits from Branch A. Also, my master shows commits from Branch A as though they still need to be pushed. I'd like Branch B to only include the changes I made since creating that branch. Then clear out the commits that are already in master showing they still need to be pushed. How do I do that? Please help.

I have tried

git checkout <branchA>

git rebase master

This was to get this branch to rebase seeing the commits were in this branch. Is there a way a git reset -soft <branchB> would remove the commits from BranchA?