There is a large variety of inks, colours, and pens. Find a pen that feels good in your hand. If you want to colour code journal entries, then keep a few pens on hand. It is best to keep your favorite writing pen with your journal so it is always handy. A journal can be anything from a spiral bound notebook, to a hand crafted leather journal.
Some people prefer to use loose-leaf notebook paper so they can organize a notebook into categories.
If you keep your journal with you at all times, it is best to find something small that is portable. If you mostly write at a designated place, then any size will do. If you use a plain notebook, do not be afraid to draw on the front, or for those of us who cannot draw, favorite quotes written in various inks, make unique covers. As your journaling experience grows, do not be afraid to experiment or change the type of journal you use. I have spiral bound notebooks sitting next to nice leather journals.