Longrass is considered an advanced crop in Ark. Its seeds are found in the wild like most other seeds, but the only way you can get actual Longrass is by growing it in a medium or large crop plot.
To get some seeds, simply harvest the various bushes either by hand or with a Dino. The seeds are not that uncommon. They can be used as a food source. In addition, they are a required ingredient in some of the Rockwell Recipes.
Longrass can be grown in any medium or large sized crop plot and will occasionally produce a seed. Keep in mind that crop plots require water and a form of fertilizer or nothing will grow and the seed will be eventually lost.
For more information on farming, please see this page: Farming In Ark