I got interested in Linux through using custom Roms and kernels on my phone.
And now I want to learn more about it.
Any advice will be appreciated

If you're brand new to Linux, a decent distro to check out if you haven't already is Ubuntu. Once you get cozy with how that works, having made efforts to learn command line, check out something like Arch. It's a more advanced but highly customizable distro from ground up with an amazing user community and fantastic documentation.

There are loads of tutorials on YouTube or blogs that you'd find from some Google searches.

Linux Format magazine is also a good read & very accessible way to learn more about the Linux ecosystem.

as @corscheid said, try out Ubuntu as it's the easiest to begin with and doesn't limit you when you get more comfortable. I started out with Linux Mint, which is based on Ubuntu and I'm still using it because it suits me. There are many desktop environments to choose from so give them all a go to find your favourite. I used virtualbox to install and test many distributions. When you find your favourite, look into what software you could use for your daily activities (email, programming etc.). That way you won't feel too strange when you start using it. When you're ready, install it on your spare hard drive, spare computer or dual boot with your current OS. You will learn commands and how Linux works by practice and googling :) hope this helps

Hello fellow ROM enthusiast!
You could start off simple. Read through the scripts that people use for rooting/building/flashing. Have a go at building it for yourself.
Go a step further and fix a warning that will show up when compiling the kernel.
You could even go off on a tangent: look into writing apps, or some tools.
My first project following android was to do a support bot on IRC for SlimRoms, which lead to doing web apps.

The world is at your disposal. Find something close to your heart, and dive in.