In relational databases I would just pop in W3Schools tutorial, install mysql in my machine and start practicing! How can I learn non relational databases in a similar way? In most tutorials I read that these databases work with multiple nodes and data centers.
Does this mean that I will be unable to learn and practice, say Cassandra, using my own single pc?
You do it just like you do it with mySQL: You set up a database on your local machine and start experimenting.
Most database systems which focus on sharding and clustering also work as a stand-alone instance. But when you want to test these features specifically, you can often run multiple instances on the same machine. When you also want to try how they behave when they run on different machines, you can use a virtualization software like VMWare or VirtualBox to set up a bunch of virtual machines and build your virtual datacenter on your desktop.
(I would recommend VMWare for business use and VirtualBox for home use)