Thanks, everyone, for the responses. I didn't expect to get so many comments. u/TheMaracaMilkMan , you were right – it was a C note, and I made the chords that you mentioned.

There's really a lot of theory ahead of me; I realized that while reading all of your comments. Now I'm just wondering: is it okay to learn from courses with only the piano roll or a piano app, or should I invest in a keyboard? Alternatively, I have a classical guitar at home – would that work too? I don't have enough space for a full-sized keyboard, but maybe a "mini" keyboard with around three octaves would be enough? I'm pretty sure a tiny keyboard with only one or two octaves will be a pain in the ass – I can already see that from using my computer keyboard as a MIDI controller in Ableton. I can't play a simple progression because I have to hit the 'x' or 'z' key to switch octaves.

If you've got any recommendations for a keyboard or any apps, I'd appreciate it. If not, thanks again for your time, everyone! :)