Try using pen or pencil with paper for a week, then your current word processor for a week, and see which feels better and which is more productive of entries that reflect where you are in your life. Either way, remember to start each entry with the date. This will become important as you accumulate journal entries and look back through them.
Just as important as being connected with your materials (manual or digital) -- or even more important -- is being connected with yourself and whatever is going on in your life. Journal keeping will help you enhance that connection, whatever method you use -- and simplest is often best.
There are dozens of books on keeping journals. Some are very good and others are copycats. I'd recommend that you start with one classic and use it for a while before branching out. One of the best, old but still good, is by Kathleen Adams: Journal to the Self.