how to get seed in the sims 4 world

You can buy a planter and buy seeds from there. Or the computer.

Alternatively, harvest the plants you find around the world and plant the clippings.

@sim31984 

@Coley666  Has the right idea about harvesting plants or buying "seeds" from the computer or by clicking on a garden planter. (Base game and beyond)

Exploring the world is a fun way to discover many types of plants.
You can gain gardening skill just by harvesting them and planting them on your home lot.
The higher the skill the more things you can do in your garden.

This list may be helpful:
The Sims 4 Gardening
Plant List, Fertilizer, and Grafting Plants
https://www.carls-sims-4-guide.com/skills/gardening/