Here are some things you need to bullet journal...
A Journal
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Make sure you find a journal with thick pages and that is dotted, or plain. A medium to a large sized notebook can if you are wanting to have charts and diagrams. I recommend the Luechtturm Hardcover or Moleskine Hardcover, these are a perfect size and come in all different colors.
Pencils
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Pencils can be used to outline your boxes or lettering until you put color on it. I prefer the Ticonderoga #2 Pencils because they erase easily and do not leave smudges.
Pens
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Make sure you get pens that dry quickly so the ink does not smudge as you continue to write or draw. My go-to pens are Papermate InkJoy Gel Pens or Sharpie Pens. Neither of these pens seeps through the page and dry quickly.
Markers
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Markers that do not bleed through the pages and have multiple uses are nice to have. Try to get markers like Crayola Super Tips or Leisure Art Pens. These pens have a sharp tip and can color easily.
Ruler
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A ruler is nice to have if you like straight lines but cannot draw straight lines even with the dotted pages (like me).
Washi Tape
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Washi Tape is used to decorate and add some pop to your pages. You can find these at craft sections, craft stores, or on Amazon.
Bullet journaling is fun and can get you organized in the way you want to. You are in full control of what your planner looks like, can add what you want to the pages, and personalize it the way you want. You can be as creative as you want to be and every page can be different. There can be pages on goals, tracking your habits, keep you in check to be healthy, and even ones that can track your moods every day. There are no restrictions.