I'm at the same expertise level as you are, this is how I started contributing to open source recently —
Contributing to open source is a hype. Do not fall for it.
Contributing to open source actually means something when you are actually trying to solve a problem — Read this article by Max Stoiber
I asked Max this very question on his hashnode AMA a year ago — this was his answer [you might want to search the question by my name, auto scroll doesn't seem to work in chrome at least, for hashnode, at this point of time]
If you do not have enough experience to be able to think and make enough that you'll actually have problems to solve, try to find github repositories / projects that you care / can care about. For example when I saw Sindre Sorhus's refined-twitter Google Extension, I could relate to the problems being solved. One particular problem / feature request that I saw in the issues was — to store a message in the text field even after the user has closed the Direct Messages modal window (right now, when you close the modal, your messages are lost and not saved by twitter). This was a pain point that I had experienced before. I could pin point the moments, when I was writing something very long and accidentally closed the modal window, only to find that all my text was gone when I opened messages modal again. So I decided to work on it
You have to find projects that you can care about, otherwise contributing could be a liability. And if it is one, why would you bother persisting to work on issues / feature requests? In other words you'll get bored.
You do not have to start contributing by writing code. NO NO NO. You can start by fixing documentation. This has HUGE implications unlike what you might think. This was my first pull request — I was following a book called Backbone Fundamentals by Addy Osmani. I was stuck for half an hour to try to understand what was wrong with my code and why it wasn't functioning the way it was supposed to. Then it clicked, the code was wrong! Imagine if I hadn't fixed this issue before someone else would have, potentially 10s of 100s of people following the book might have been confused. This is the power of open source, you can make difference fairly easily.
I made a small video the other day about how I got to work on a small pull request on a fairly large open source project. The pull request got merged today! (Ah that feeling that your code is actually running live!)
A good way to start finding good first issues is by going to the issues of repositories and filter out issues by 'Good First Bug' label. For example the project that I contributed to (spectrum.chat) has a lot of 'good first bug' labels on issues
Do not hesitate to ask for help from the open source community (if you see the video I posed above, you'll see that I was actually helped by three different people who helped me know everything I needed to know to work on the pull request. So you need only ask). That said, you should do your homework before asking anything. By that I mean you wouldn't do any good to yourself or others if you ask questions like — I'm a newbie / junior developer and I want to contribute to open source, could anybody help me. Aaaaaeeee (Buzzer). You should dig yourself into code / docs first. Try to understand. When you get stuck, you ask. But first you need to know that you are genuinely stuck and not impatiently giving up.
Hope this helps you do the right thing! If you think you are not experienced enough, first learn things by following different projects / repositories and slowly start checking out issues / feature requests and then slowly start contributing. What's the rush!
In short in order to start contributing to open source, you gotta learn things! That's it. It's all about learning. You'll be easily able to start applying afterwards. Be curious and keep learning, that's the shortest way to contributing. Keep you fundamentals clear and ride on!