You can add gitignore later, the problem is that if you already uploaded files that should be ignored, it's a pain to get them removed from the repository. It's best if you handle the gitignore from the very beginning. Make sure to remove videos - those are huge and nearly useless on git, and will very quickly eat up your available space on GitHub.
(I accidentally uploaded a 10 minute video to GitHub at work one time. Nobody was able to access GitHub within the office for ~15 minutes while it uploaded. It was embarrassing to admit that I caused that problem. So consider yourself warned now).