Before knowing how to contribute let me tell you what is open source. Although Open source word is it predefined, but i will tell you a bit about this. It is some code base which is open for anyone to use or to contribute in that.
One of the most popular place for open source is GitHub, its a code hosting platform for collaboration and version control.
But before learning about git and github i belive you should first know how to find the open source projects and for that Google Summer of Code can be a really good website you can go there and can find lots of projects (Here is a video about how to find it: Beginner's Guide to open source) ©CodeForCause
And for contributing to open source you need to learn git and github (Git and GitHub Tutorial) ©CodeForCause
If have watched the above two videos then you must have got a better understanding about Open Source and contributions. Now fast forward to when you have found a project then go in issue tab,
there you need to click on lables and you can search for good first issue.
After finding an issue now you can comment below that issue. The community is really helpful and they will be more than happy to help a new contributor in starting out and then you can directly see which folder has the issue, after that try to understand thing in that folder, don't worry much about other folders and run that project in your local environment(in your computer).And that is pretty much about it. For more guidence you can watch Code for Cause Youtube Channel.