Journaling (or journalling - we can't decide!) can mean different things to different people. It can be just noting down the things you do each day, even if it's just a couple of lines. It can be more creative writing about your thoughts and feelings, and it could include your plans for the future. It can also be a way of planning the things you're going to do, as well as the things you have done.
For some people, it's an artistic or at least creative outlet, marking out the days with beautiful calligraphy and sketches or even paintings of the things they've seen. For them, a journal will need good quality paper, and probably needs to be a good size. If that's you, you'll need brush pens, and maybe watercolour paints; fine and broad pens for a variety of lines; and probably a good selection of coloured pencils or pens.
For others, the decorations are a bit simpler - there to support the writing or planning, to make the whole thing look a bit more pretty. A few different rolls of mt tape can make decorating pages easy, and a selection of pastel-coloured pens and highlighters can give everything a more elegant look without too much trouble.
Some people don't care what it looks like - it's all about the content. They might not be so concerned with colours, but may still need a few different pens to indicate different types of information. If you're one of these people, you just need to find the pen you enjoy using, and that's comfortable for you to get your thoughts down with the minimum of fuss.
If you're planning to take your journal everywhere with you, you'll need it to be portable enough. If you're going to leave it at home, a pocketable notebook could be a good idea to keep some notes when you're on the go, so you don't forget things. It could be as simple as a couple of index cards in a pocket, or one of any number of different pocketable notebooks.
However you want to use your journal, you should find what you need in the sections below.