If you want to keep a sketchbook there are no rules...drawing is not like riding a bike, you need to keep practising or your skill level slips. Doodling in a pad, collecting images that inspire you, testing colours, copying artists' work or experimenting with materials are all great ways to develop your artwork; instead of buying a sketchbook you might decide to make your own or find an old book to draw in. If that all sounds a bit much, just keep a small pad with you to sketch in rather than scrolling on your phone all the time.
The themes below are roughly divided up into areas of study required by examining boards for most art qualifications...research and investigation into artists, a personal response to their work, experimentation with materials, development of ideas, drawing skills, and analysis of your own work in the form of annotation. Whether you are studying art or just looking for a hobby, following this learning path is hugely worthwhile; looking at art is the best form of inspiration, it doesn't matter what age you are and immersing yourself could take you all the way through art school to becoming an artist.