I really love both digital and traditional writing because they have so many pros way more pros than cons. They are both really cool ways to really express yourself and to let the other people in your life know what it is you want to say exactly to them no matter what it is either good or bad. It is extremely healthy for ya to express yourself.
Just write about whatever comes to mind, how you day went, people you met, how you felt, etc. definitely go with pen and paper, i've found it easier to complete goals when you have a tangible object constantly reminding you, and your notes app just doesn’t cut it.
I write on topics that I know about and/or have experienced (also, things that pique my interest that I’d like to learn more about). I also keep a running list of topics/jumping-off points. Starting is often the most challenging part!
I think each person finds their own way of journaling. I tried paper and pen and hated it. I felt tied to using the same pen (stupid I know). And it was hard to keep the journal where I could write in it daily even if I forgot to do it before I left the house. I didn’t want to always be carrying it around with me. So I started using the notes app on the iPhone. I just write each day something. Sometimes it’s deep thought, sometimes it’s “hey I’m journaling today”. But I haven’t missed a single day of journaling in 40 days so something is working!