Create repository

cd path_to_central_reposity_folder_on_usb_stick
git init --bare --shared=all

Clone repository

cd path_to_where_i_want_my_files_on_computer
git clone path_to_central_reposity_on_usb

Commit changes

git add -A
git commit -m "Made some changes!"

Push changes to USB stick

git push origin master

Get changes

git pull origin master