For you to build and customize your menu, please refer to these instructions
Regarding to your login issue, what is the error message when you try to login? What I’m thinking is there might be some issues in your browser and you might need clear your cache, please refer to this – https://en.support.wordpress.com/browser-issues/
Or if you have issues with the password, please refere to this – https://en.support.wordpress.com/account-recovery/
Let me know how it goes ok?