The serrated leaves mark the start of the vegetative stage (informally, for me anyway.. The whole thing is more of a gradual slide than a sudden transformation.)
The small round leaves are called cotyledons. They are not actually leaves but leaf-like nutrient reserves that fuel the development of the seedling. Once these have been depleted, they'll wither and die (maybe that's a better end point for the seedling stage. By this time the first true leaves will be strong and healthy and your plant will be ready for light fertilizing. Start as minimally as you can so you don't overload the plant. 1/16 or even 1/32 strength would be even safer, much better to be conservative here, at this point nutes are not necessary, and could be harmful if you're overzealous. As the plant grows, gradually increase the ratio. Find some good picture of nute burn and learn what it looks like. Watch your plants carefully, and as soon as you see any signs of it, skip a fertilizing and scale back to a known safe amount, unless it's real bad, (which it won't be of course, because you've been increasing their nutrient ratios in slow, small increments) then you may have to flush some out.
As for watering, just make sure the soil dries out between each watering. This is another situation where you have to be seeing your plants every day, and become familiar with the individualities of each. Again, find good pictures of overwatered and underwatered plants to have some idea what you're looking for.
You can also get a moisture meter, or just lift the pot when dry and again when saturated. Check the weight whenever you check them, and if they feel light, probably a good time to water. Every 2-3 days is what you'll probably end up doing, but don't fall into the trap of watering by the schedule without input from the plants.
