Don't despair there is help for your potted lemon tree. Often a newly transplanted lemon will drop leaves that turn yellow as it makes new roots. Do not overwater or let this plant sit in drainage water. Lemon trees do need bright light or the leaves will yellow so it needs to be near a sunny window or greenhouse. Too much water and poor drainage are other causes of yellowing leaves. I will assume you used a new container with a drainage hole and that you used potting soil not dirt from your outdoor garden. Lemon plants also need iron and nitrogen to grow green leaves so be sure to add a slow release plant food like Osmocote to the potting soil every six months. One more cause of yellowing leaves is spider mite so look on the back of the yellowing leaves with a magnifying glass for tiny spider like critters and even some webbing. If you see mites wash the leaves with a soft cloth dipped in a mild soap or spray with an insecticidal soap sold for houseplants. Before you water poke a finger into the soil and water only if the top one inch of soil is dry. This will help you to learn when the plant needs water and to not water too much. I see lemonade in your future, Keep growing, Marianne Binetti