Don't know where to begin?

While ultimately it doesn't matter which arc you begin with, Cecil arc is recommended if you want my recommendation.

And then Masu arc + Shattered union arc. 

Feel free to visit the lore section as well which holds stories of its own.

Shell of Pluto arc (2)

Shell of Pluto's story.

[Shell of Pluto] [1] [Shell] [9614]

[Shell of Pluto] [2] [To Pluto] [9649]

Shell’s home coming to Pluto at last. Ivan makes his debut here as well. He is the son of Admiral Gabrio from [Shattered union arc] [2] [Karin Bau] [9603].

Shell herself has some connections to Ashuta. Read [Ashuta arc] [1] [Ashuta’s blade] [9599] to find out.