Go to Jira and select an issue/feature you want to work on from the board. Click on it and you will be presented with the detailed view. Find development option from the very right panel of this detailed view. There you will find create-branch command. Copy the command.
Open you projects' root directory in terminal and make a pull git pull. Make sure you are running pull command from within main branch. Then paste the command you copied from jira ( it should be something like git checkout -b <branch_name> ) and hit enter. Now you are in your new branch where you should code for that issue/feature only.
Once you are done with coding and testing. This is the time to let the reviewers know. So you will have to push your local branch on remote. To do this follow these commands:
git add// adds a file in staging area.