Like many things in life including being a practicing pagan, you can spend as much or as little as you like on materials and tools.
For many years I used mostly just a black pen. Limiting what materials you use often helps with creative block too, as you might be able to force yourself to use those materials in new and interesting ways.
Necessary materials are very basic!
A pen or pencil: I love thin Sharpie pens, but be warned: if the paper is thin, they tend to bleed. I also like these gel pens too.
A book - lined or plain notebooks are the typical choice, but I have also journaled in altered published books, where the text became part of the artistic process. These sketchbooks from Daler and Rowney are absolutely my favorite. The paper is thick and can be used for painting, collage, sewing and sketching. The ivory color of the pages is very attractive.
Ephemera - pictures/text cut from magazines and newspapers, old photographs, NEW photographs, postcards, wrapping paper, ticket stubs, receipts - you get the idea.
Markers - Quality varies, but that doesn’t even matter for journaling in my opinion. I have had pleasing results with washable markers for children. Illustration markers have a nice fluid texture, but again can bleed. These ones are cheap but I use them quite a bit.
Wax crayons - I use my daughter's crayolas quite a bit, they are bright and colorful. Wax crayons are versatile and there a many interesting effects you can achieve with them.
Paints - I like guache for the vibrancy and the fact that they can be mixed easily with water. These ones are reasonably priced and a good small set to get started. I also use children’s poster paints - they are vibrant, are easy to work with and are quite cheap.
Gel pens - I have a set that was inexpensive with many different colors. If I can't get to my art space then these are what I fall back on. I have to hide them from my daughter though!
Glitter glue - So fun to use and really looks magickal. Takes a while to dry so you would have to keep your journal open somewhere to stop the pages sticking together.
Washi Tape - Love this stuff to make borders and labels. This set was cheap and a lovely mix of designs.
Patterned paper - For collage, and more. I have a weakness for Japanese designs but these are great as well.