Is it a good idea to start playing with open source projects for beginners?

The question makes no sense at all, because you did not tell anything about the goals of the software you need to have.

As you mentioned that you want to "create" software, it sounds like you would need to find open-source software to learn how to do programming (otherwise you would know pretty well "where to start") and to use fragments of this open-source software for your own work.

I would say, this is not a bad idea, but a very bad idea. People usually successfully modify open-source software only when they know how to make it better. It happens when you would say: "some good work is done by those guys, by in many places, I know how to do it much better" or "I have good idea how to add important features to this work". Then you can reuse some work which already has been done by others, and create your derived work. This way, you collaborate with people you never knew before, and create better work.

Usually, you publish your derived work. It really depends on the license. Some licenses requires you to provide all the source code of your derived work, but this is not fully "free software". Really free software allows to use derived work for whatever purposes, commercial or not. There is a lot of discussions and controversies around different open-source licenses, but I mentioned this only to bring to your attention that you also need to pay attention for original open-source licensing, without discussing this aspect.

This is how open-source really work; and this is how many people can benefit from sharing results of efforts of creators of open-source software.

As you are the one who does not know "where to start", it's hard to believe you are strong enough to contribute to open-source software; at best, you would be able just to use it. It means that "playing with open-source projects" would be counter-productive. You can "play" with it, now what? If you really need how to develop software, you need to start writing your own software. On this way, looking at already developed projects can only distract you. Aren't you the one who is hoping to use the "easy" way? Then be aware: "easy" ways goes nowhere. If you need to learn successful development, you need to start with very simple projects but develop them to perfection, and get to more difficult projects as you gain confidence. You cannot create useful output for small business, which means commercially valuable production, if you are still at the stage when you have to ask questions the way you do.

—SA