First, it is much easier to keep a diary if you LOVE it and your writing utensils. Choosing materials that you look forward to using will make you WANT to use them on a regular basis.
This is my current diary. It is just for writing (aka no media). I made a goal to write 1 page every day about being a new mother. This makes my diary relevant and makes it easy to track my goal.
Choose your diary: it does not have to have lines! You can use a sketchbook if you like to draw or paint or want to add lots of pictures or magazine clips.
This was an all-photo diary I made to collect cool magazine articles and photo collages that I made that were personal to me. I did it slowly over a few years.
This was a combination written and media diary. Some pages are all pictures, some are all words, and I even threw in some Star Wars stickers just for fun! It took me about 2 years to finish.
This is my wedding diary, it started as just a photo scrapbook of things I liked from wedding magazines, but I ended up putting in my wedding photos, cards, stationery and honeymoon mementos!
It also helps to have a writing utensil you really like. I love to go through the office supplies at target but you could try a specialty store like michael's for fancier supplies and stickers, etc.
If you want to make writing a habit, do it at the same time every day, like right away when you get up, or right before bed. Start small, and write about a specific topic each time.
Then when you get more comfortable, you can write more each time. Try to remember your dreams, write letters to your future self, try writing a short story- anything that helps you relax and feel good
I think it also helps if your goal isn't to get to the LAST page of your diary, but instead, to try to use each page the way you want, which will slowly but surely fill the whole book!
Take your time, be creative and don't worry about doing it "right"! It's your diary and no one else's! Put your special touch on it, don't try to do it how you think it SHOULD be done!