Seeds are fundamental for farming in Ark. Without these, you're going nowhere. Finding seeds is easy, however, but collecting them effectively and in decent numbers can be a bit of a challenge. Luckily, there are a few tricks of the trade that can make it quite easy.
To harvest seeds, all you need to do is walk up to any bush with either your bare hands or a torch equiped and press E. Keep on going and you'll get berries, some fibre, and the odd seed drop. If you do this long enough, you can get all the seeds that drop from these bushes. This will take time and patience.
The fastest and easiest way to harvest seeds & berries is with Dino power. You'll want to have a Dino with a good strike area and some melee damage. A Trike or Triceratops is the best early tame for this purpose. Its huge head hits within a good radius and will allow for quick collection of berries and seeds. In half an hour it's more than possible to have tons of berries and at least a few of each seed type.
But for best results you'll want something bigger. This isn't necessary by any means, but for the sake of argument a Stego or Bronto really harvest quite rapidly. Just be careful with the big tails. They can hit a very wide area and it's easy to aggress nearby Dinos or damage structures. They will also bring in a small amount of wood and good deal of Thatch.
You can find seeds for most things by following the tips above. Below is a complete list. Keep in mind that you won't find Plant Species X this way, and you'll only find Rare Flowers in certain areas. As for Rare Mushrooms, those are another story altogether. I've got links for these special cases:
Plant Species X
These are the crops you can grow from seeds you can find. This list does not include the special cases listed above:
There are also seeds that you can collect without ever finding the vegtables they produce. To get Citronal, Longrass, Rockarrot or Savroot you need to find the seeds, like the other seeds, and then grow them in a crop plot. For detailed information on farming, please check this page: Farming In Ark