You can transplant them in bigger pots already, the yellowing is likely a combo of overwatering and root limitation. Perhaps the soil lacking something or being too hot.
You can get a bag of certified organic potting soil to start off with, or any organic soil you can find that’s decent.
Keep them on the windowsil in sunlight as much as possible, I noticed they grow faster in the sun.
I’m experimenting with compost towers now, should be pretty fool proof, throw in kitchenscraps (especially bananapeels) and pour water down the tower so the topsoil stays more dry, that prevents insects like fungus gnats laying eggs. 
Check out my journal for more info if you want a very easy way of growing. 