How you want to use your journal will help you decide what journal is right for you. If you want to paint directly into your journal the paper needs to be watercolor or another heavy paper type. If you want writing and photos you could go with traditional line paper or grid type paper. You can either make your own journal, purchase one, or paint on art paper then paste it into your journal. Stickers and photographs are also great options!
You can buy a sketchbook, notebook, or even a planner to work in. The size is purely preference. It depends how much time you plan to work on your journal. Smaller pages will be easier to fill up with text and pictures. Larger pages will have room for more, but take more time. You can find these items at book stores, retail stores, and art supply stores. You can buy any lined notebook or graph paper notebook to use.
If you're looking for something made beautifully for journaling check out these journals. The links will direct to a blog showing off features of the journals.
My personal favorite is the Hobonichi brand.
Prefer digital method? Check out these articles
My favorite app for writing - Good Notes5
My favorite planner app- Notion