If you want to give a best speech on topic ENVIRONMENT you should remember following points :-

1) You should give a proper start of your speech
2) The speech should be medium not too long
3) You should mention all the important points in your speech
4) You should tell about importance of environment in our life 
5) you should prepare an easy understandable speech.

i will give you a speech which helps you in your work 


All the things which is around us known as environment
Environment is the surrounding in which we live. It is the source of life. Our whole life is dependent to the environment. It directs our life and determines our proper growth and development. Bad activities like pollution and uses of polythene is directly effect our lovely environment. 

The need of human beings for food, water, shelter and other things depends on the environment around us. So it's our duty to make our environment clean and pure.  
Some of the environmental problems are rapid growth in world population, deteriorating natural resources, diminishing forests and wetlands, erosion of soil and coral reefs, depleting underground water, regular shortage of fresh drinking water, vanishing plants, salinization in Africa, Asia, Latin America and Middle East.

It is our responsibility to save our environment and earth and make the possibility of healthy and happy life here.