Working on open source projects requires absolutely no qualifications. Provided you can do some programming, and make a Pull-Request (Git), that's all you need.
And let's face it, doing work on some open source projects is a great way to build your résumé without having any experience as a hired engineer.
I assume you have a LinkedIn profile already. But your Git profile is more often than not even more attractive if you've been actively contributing to projects. It will show that you've made meaningful contributions to codebases, and serve as a portfolio of your accomplishments.
Getting started with contributing:
First off, you're going to need a profile on https://github.com/
By using the Explore link, you can start finding projects by field.
It's going to take some time, but in order to make a meaningful contribution, you should use a program, get to know it, and see if you can think of a way to improve it.
Making a pull request:
When you've stumbled upon an avenue for contribution, you make what's called a Pull Request (PR), whereby you clone a project (i.e. download all source code. There is no "request" here per se, because it's open source. Anyone can clone the code), make a branch, commit code to that branch, then you request a merge into the master branch. The code will then be reviewed by the owner(s) of the project, who will either accept or reject your changes. Don't be discouraged by rejections; it happens. Either your changes pose an undesired change, or they simply disagree with how it was implemented. More often than not, the owners are nice people, and will let you know what's up, so that you can eventually make the contribution after all.
For more info on making a PR, see here.
Personally, I've only made Pull Requests to private repositories for friends, but I have experienced someone making a Pull Request to one of my own public repositories, which was nice.
Create your own repositories:
If you're having a hard time finding ways to contribute, it's safe to chalk it up to lack of experience. But getting experience can be simple! Create your own project!
It can be absolutely anything. Think of a program or a website you'd like to make. It can be something as simple as a website where people can convert between different units of measurement. Yes, it already exists, but there is still experience to be gained here. And as you work on it, you can make it better and better, until you've made one of the best unit converters out there. And I promise you this; as you do this work, you will come up with more interesting ideas, and be in a great spot, because now, you know how to get started with a project!
There are plenty of guides out there on how to start a project using Git, and getting to know Git is something you put on your CV, because it is arguably the most prominent version control system out there, used by everything from one-man projects, to huge enterprise systems. It's gold!