An eco system is a collection of things that together can form a balance and survive without outside help. This collection of living things will work together to florish. Making a small eco system is easy and very educational. The small contained eco systems are called eco-sphere. You can make one really easily and it should require very little maintenance. I did this the following manner:
Put your eco-sphere in a nice place where they will have daylight. This is needed for the plants to make oxigen and to grow. Do not place the eco-sphere in a place where it will be hit directly by the sun. This will heat up the water way to much and you will end up with cooked shrimp. It there is to much light entering your eco system you should notice an abundance of green or brown algea. This tells you you will need the eco-sphere to a place where it is a little bit less bright.
The shrimp can eat the algea that will grow in the eco-sphere. The snail(s) will also eat the algea and everything else they can find. The snails should also keep the glass clear. The poop from the snails and shrimp should function perfectly as food for the plants, and thats it. You just made yourself a really nice eco system!
You can also try an eco system without so much water. You will then need to add some dirt and some plants. I tried this a couple of times, but after a while the plants just got to big or simply died. Making a water based eco system is much easier.