Well, local comic bookstores might have it. Also, try looking on Amazon or other major online book retailers. Sometimes libraries also stock popular manga like JJK.
The recommended way is to begin with Volume 1 and follow the numbering. This ensures you follow the plot's development and character arcs properly. Some people might jump around, but you'll get the fullest experience by going in order.
First, you should be really familiar with the JJK universe. Read the manga, watch the anime. Then, pick a character or a group of characters you want to focus on. For example, if you like Gojo, you can start thinking about a story centered around him. Next, come up with a simple plot idea, like Gojo going on a solo mission to defeat a powerful curse. After that, start writing an outline of the story, including the beginning, middle and end.