To get seeds you have to go to Vesta's farm and you can buy seeds from anybody there. Make sure that you know that it is the right season to plant a certain seed. Vesta will usually tell you what seeds do good this season. Then plant all of your crops in the farthest dirt area from your house. To plant one you have to till the square with your hoe then you have to plant the seed and water it once a day. the other two dirt areas are for planting trees. That's about it.
Here is some info. about the seeds!
TOMATO: Spring-Fall COST: 30g
WATERMELON: Spring-Summer COST: 60g
STRAWBERRY: Fall-Spring COST: 30g
MELON: Summer-Fall COST 50g
TURNIP: Summer-Winter COST 20g
POTATO: Winter-Spring COST 40g
CARROT: Fall-Winter COST 30g
SWEET POTATO: Fall Only COST 40g
FRUITS MAINLY GO YEAR ROUND ON A TREE, MAKE SURE TO PUT THE FRUIT TREES IN THE FIELDS WITH THE SOIL THAT IS NOT AS GOOD