The default way to copy/paste in Git Bash is by using the menu (accessed by going to the top left) to "mark" to select what you want to copy and then use the menu again to paste.
This gets inefficient pretty quickly, so it's a good idea to set up QuickEdit mode to make copying and pasting much faster. To do this, go to
Menu->Properties and in the
Options tab select
Then you can copy text by selecting it and then right clicking, and you can paste by right clicking without anything selected on the screen.
If you're having trouble, you can find more detailed instructions about how to set up QuickEdit mode here. Those instructions are for the Command Prompt program, but they will also work for Git Bash.
For copy and paste information using Git Bash for Windows with mintty terminal emulator refer to documentation section of mintty.