At first reading the Autobiography seems pretty straightforward. Franklin's not a fancy guy, and he's self-educated. It's not like the book is full of Latin phrases or peppered with allusions, like the way T.S. Eliot writes. But we don't really talk the way Franklin does any more, and getting in the headspace of an eighteenth-century diplomat can be kind of tricky. It Seems like Franklin Capitalizes lots of random Words, which we Frown upon Today. (See what we did there?) So, getting used to that takes some time.
As we read, the book gets more complicated structurally, too. On a larger scale, Franklin's mostly sticking to the order of how things happened in his life as a way of guiding the narrative, but he jumps around in time, mentioning specific events out of order. That can be kind of confusing because the book's not super-organized. Sometimes, it's more like listening to someone thinking out loud and telling you about his past, then it is like reading an organized, dry document with a standard beginning, middle, and end. But that's part of what's so cool about Franklin's Autobiography – who wouldn't want to listen to the recollections of someone so interesting?