Many a house plant has been loved 'to death.' Overwatring. Stick your index finger into the soil down to the first knuckle. If the soil feels damp, it does not need watering. Dry, water it until the water starts to come out of the bottom.
Keep in mind that Rosemary is very invasive. When planted in the same pot with another plant of a different species, it will take over the space with its roots and kill your other plants in the pot. Rosemary should be planted alone, in a pot.
When you get a 2" potted plant home, let it sit for a couple of days to adjust to its new climate. Check to see if it needs watering. After a couple of days, transfer the plant to a 4" pot. Planting it in a bigger pot, will allow it to grow more roots and nothing on the top. Gently knock off the soil from the 2" pot and place in a new pot with all new soil.
To keep your herb plants producing those tasty leaves, when you see a flower developing at the top, pinch it off. The plant is getting ready to reporduce and then die.
Some plants do very well in shade, others with full light. Google the type of plant and see what growing conditions are best for that plant. If you are going to plant your seedlinigs in one pot, make sure they all require the same growing conditions.
Make sure you have a catch tray under your pots to catch extra water that comes out of the bottom. Empty the tray. Do not allow the pot to sit in the water.
This is all I can think of right now. Good luck.