Chenjia, great advice from Backyard Gardener.
I am guessing you started the seeds in the potting soil.* If so, your young plants may have experienced fertilizer burn.* Most potting soils today come with slow release fertilizer already mixed into them.* If you added more fertilizer, and even if you didn't, it could have been too much for them.* Seeds should not be fertilized until the second set of true leaves appear in the seedlings.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.