First thing you need? A journal. You can buy a nice one, or use a cheap one, or use something you already have. It doesn't matter at all. Some people like to carry their journal around the garden...if you do make sure it is water resistant! I prefer to doodle in mine before bed, so it doesn't need to be particularly tough.
hat should you write in your journal?
Thoughts on your world view and deep emotional journeys? Not so much. Record when the first daffodil pokes it's head up from the ground (right now!!) and when the first crocus blooms. Record exactly where you sowed a few seeds or where you transplanted those day lilies. It doesn't have to be pretty, just factual so you have a guide the next year. If you have a late frost or a crazy windstorm, write that in there. You don't have to write everyday if you don't want to. This is not your life, it is just your garden. This is supposed to be fun, so if you hate it, stop and find a new hobby. Give it a try for at least a summer though...you'll probably be hooked :)
Here are just some examples of what goes into a journal. A primarily lists, crossed off lists and sketches, lots and lots of sketches, lists, sketches, and etc.
After you have your journal, it is time to think about your grand plan. This is the best part and the hardest part. If you are really into designing a garden space, this is the time to go to the library and check out book after book about gardening. Check out books with lots of pictures and books with just tips on how to be a frugal gardener. Check out books about garden styles and how to compost. Learn everything you can before you design your master plan.
Once you have a record of what is going on in your garden, a vast knowledge from all of your research, and a hardy little sedum growing out there in your yard, it is time to sketch a few plans. ...and then you get to be a Senior...it's so fun!