Ano Hana, is there a origin point?

For Ano Hana is there like an origin for it? Like a LN that evolved into an anime?

I'm watching the first episode and right now it seems really confusing to start, and I was afraid I'm watching a sequel and I may spoil myself.

The whole title is;
Ano Hi Mita Hana no Namae wo Bokutachi wa Mada Shiranai.

Thanks for any responses in advance.

There was a light novel before the anime, but it was a month before airing the anime.

Anime original. The novel is an adaptation of the anime, written by the same screenplay writer of the anime. It just happened to start serialization before the anime aired.