Start with your interests. Do you enjoy cooking, decorating, fashion, humor, weight loss, excercise...then when you see a great idea or recipe, you can pin it on a board set up specifically made for that category.
I love to cook but didn't previously have a great way to keep all my recipes organized, most of them I found online so I had about a million bookmarks and the recipes were very hard to find. Now, I have an appetizers board, a dessert board, a beverage board...everything is nicely organized and I can spot the recipe I am looking for easily by the picture. I also have a humor board, a music board, gardening, and many more.
I use it constantly but mostly for cooking.