Well, one method is to check out fanfic review blogs or websites. Some bloggers dedicated to JJK will review and rank fanfics. Also, ask other JJK fans directly. You can go to anime or manga conventions where there are likely to be JJK fans. They might be able to point you to some amazing fanfics they've read. Additionally, explore the related tags on fanfic platforms. Sometimes, the less popular but really good fanfics can be found this way.
Well, 'Yuji's Ascent' is a top - notch JJK fanfic. It follows Yuji's journey in a more personal and detailed way. He has to face new challenges and grow in different aspects. Then there's 'The Cursed Family'. This fanfic is interesting because it creates a whole new family dynamic within the JJK world. Also, 'Satoru's Redemption' is quite good. It gives Gojo a new arc where he has to deal with his past mistakes and strive for redemption.
I'd recommend beginning at the beginning, so Volume 1. That way, you get the full story as the author intended without missing any crucial details or build-up.
I'd suggest beginning with the first chapter. It sets the foundation and introduces the main characters and the initial plot. From there, you'll be able to follow the story's progression smoothly. Also, check out some online manga platforms that have clear starting points and good reading interfaces.
One great opening line could be 'In a world where the sun never set, the people had forgotten the beauty of the stars.' It immediately sets an unusual and somewhat mysterious setting that makes the reader wonder why the sun never sets and what that means for the people. It also creates a contrast between the sun and the stars, which can be explored further in the story.