I'd recommend beginning with the prologue or the introductory chapter. It sets the stage and gives you a good foundation for understanding the characters and the world of the manga. Sometimes, there are also special editions or prequels that can enhance your initial experience.
In the story, lavender and lilies could be symbols of the ghost's past life or attachment. Maybe the ghost had a special connection to these flowers when it was alive, like a gardener who loved to grow them or a bride who carried lilies on her wedding day.
There are several options. Some people find it on dedicated manga websites. Another possibility is to check if your local library has digital access to such manga. Also, some subscription-based manga apps could have it.
Some good places to check for 'Loving Yamada at Lv999' manga are dedicated manga apps such as Comixology or Webtoon. Sometimes, libraries that offer digital manga collections could also have it.