Buy yourself a new notebook that you like and reserve it for the specific purpose of journaling.
Buy a pen that you'll use only for journaling. Although any old pen will do, it's nice to have one that will only be used for this specific purpose.
Find a private place where you won't be interrupted and can focus on your writing.
Write whatever comes to mind. The great thing about journaling is that there are no rules! The purpose is to write whatever you want to. Nobody else needs to read your journal so you're free to write about whatever you want.