Even without an instrument, you can train to develop relative perfect pitch (not actual perfect pitch, only children can learn that) using this website:https://tonedear.com/
You'll want to start with the intervals quiz; don't bother with any of the others until you know all your intervals. An interval is the pitch difference between 2 notes. A scale, like the ones you can select in beepbox, is just a collection of intervals coming from whatever note the root is, i.e. the "Key" select in beepbox (not technically accurate because a key is both a root AND a scale, for example G major has the root G and the intervals of the major scale).
Look up what the diatonic scale is, and learn what its "modes" are, especially the major and minor modes (sometimes called Ionian and Aeolian). Do the same for the pentatonic scale, and understand that it is just diatonic with certain intervals removed.
As for harmonies/chords, those are also connected to intervals. The major chord is made of a root, major third, and perfect fifth. The minor chord is a root, minor third, and perfect fifth. You will understand what this means once you learn the intervals. There are probably millions of possible chords, almost all of which you will never need.
If you any questions about any of that, or about other music-related topic (rhythms, terminology, whatever), just ask. I am bored and on break, so I will probably watch this thread.
Also, the music theory I just wrote out should be very helpful, but the best thing you can do is start practicing right away. You can use your voice if you have no instrument (I don't usually do this because I don't like singing, but I did it in high school band and it really is helpful). Practice playing scales, look up exercises online, and most of all, improvise.