That artifact seed gives you a random miracle or better item, with I think a chance of one of the set artifacts. KEEP IT!!! Just store it in your house.
How?
Here's some info on planting and seeds:
1. You do not actually need a farm to plant seeds. You can plant seeds straight in the ground in your home. Like, even the pavement. To do that, press the "t" button while standing on the spot you wish to plant.
2. Seeds are edible an thus can be easily and indefinitely stored in your house's fridge.
Now, you should save all seeds until you get the gardening skill. Do not plant seeds right away! Save them up and save up some platinum coins.
When you have enough platinum coins, you should go to the trainer, buy the gardening skill and then use the remaining platinum coins to raise the skills potential to 100%-200%. If you don't have enough platinum to get it to that level, get more platinum.
Once you have over a 100% potential in gardening, planting each seed will train your skill by 0.600 or more. First plant all your vegetable and fruit and gem seeds. This should raise your gardening skill fast. If at anytime your potential in gardening drops below 100%, go raise it with platinum.
You can start planting your artifact seeds only when you reach level 10 or more in your gardening skill.
To get more seeds, consider converting to Kumiromi of the Harvest. Worshippers of Kumiromi can convert rotten food to seeds (all rotten foods, not only plants). This happens automatically when the food rots: you should get a message like "3 grapes rot. You harvest 2 seeds" or something. This process is random: not all food will be converted to seeds. It seems that the chance of you harvesting seeds raises with time, which may mean it is dependent on your piety.