I was once a member of the Royal Botanical Gardens - I love plants. I enjoy the outdoors - but prefer gardening inside where there are less problems. That said - everyone who tries to keep a house plant runs into the situation where the plant starts to droop or lose leaves. Sometimes this is from under watering - stick your finger into the soil - down to about your first knuckle. If its dry - then you need to water.
Over watering can be just as big of a problem - some tropical plants like to get very little water. I have seen many people bring a plant home - water it - have it go into "shock" and they just end up throwing it out. Patience is a necessary component to indoor gardening - with no direct light things move slowly.
If your leaves are dropping, your plant looks like its dying - and If you have never fed your plant before - this is your most likely culprit. Re-potting would be another option - but that is a subject in and of itself.
Feeding your Plant
My lemon tree has seen better days. I just harvested the lemons from it and have brought it indoors for the winter. This guy needs more water than I give him , and I wasn't faithful in feeding him (they need 4x a year feedings). Luckily my tree will be more resilient than most house plants - I will bring him back and he will move outside at the end of May. Leave all the leaves on the tree that haven't fallen yet (don't go ripping them off). Most plants can still live in this situation - but think of leaves like solar panels - even if there is a thin strand connecting it to the plant there will be energy. Pickup the leaves and debris from around the plant and put it to the side.
Get yourself a watering can - it helps to know how many litres/gallons the container is.
Use a two part A/B liquid fertalizer if you can - they can be found at a local hydroponics store (best results). In a pinch"miracle grow" from your local hardware store will do.
Fill your watering can with a bit of water, then add the nutrients as per the manufacturers instructions. Finish filling up with water and you are ready to go.
If your plant is bigger - try to dig a hole and bury some of the dead leaves - it will compost and help your plant in the future!
Water your plant in a circle from the outside in. Try to avoid pouring too intensely as you could expose/damage the root system. Its best to buy a cheap watering can (plants are much more expensive),
You will know the plant has had enough water when you see some in the drip tray (or it seeps out the bottom holes of the pot or overflows). Put the plant in a sunny window sill - it will need lots of light and should remain moist especially while it is generating new growth. Try to repeat this procedure 2-4 times a year.