The 1st comment is correct, I would also like to state, that:
1. you can obtain seeds by finding them around town or finding them with your pet dog (digging).
2. You can upgrade your gardening skill by gardening, watching the gardening channel on tv (your tv depends on channel variety), or reading a book on gardening.
3. your traits/lifetime happiness traits may affect your gardening skill, such as green thumb
4. Seeds have different rarities, depending on your skill, you cannot plant rarer seeds