Design a poster for promoting awareness in the need for conserving nature.

There are a few things to think about when designing posters, regardless of the topic.
No matter how big your poster is, don’t try to cram too much information into the space. Your design should be easy to decipher from a distance, and it should get the message across as quickly as possible.
Most people aren’t going to stop to read through long blocks of text or study a lot of confusing images. Imagine that you’re designing a billboard, and your viewers will only have a second or two to look at your work—your message has to be clear and memorable. Figure out the main point that you’re trying to make, and come up with a simple design around that one idea.
An appropriate graphic and a few well-chosen words can convey a lot of meaning. Include a “call to action”, such as an invitation to visit a website for more information. If your design sparks their curiosity and concern, they’ll follow your link. (That’s where you can present them with all the details that you had to leave out of the poster.)
Here’s a page with a lot of good examples of successful advertisements and posters: http://www.creativebloq.com/inspiration/print-ads-1233780)

While I can't exactly design a poster for you, I can definitely help you come up with some ideas! 
First things first, I would say to take a look at the area where you live. Is there any specific issue you see in your hometown in terms of conserving nature? Here are some questions you can ask yourself: 
Are trees being cut down in my area? Has the area I live in already been deforested? How does deforestation affect other living things in an environment? 
Is the area I live in affected by pollution? How does pollution have a negative effect on different types of environments? (water pollution, air pollution, etc.) 
Are there any animals in my area that are considered an endangered species, or that are having their natural habitat destroyed? 
Are there any natural resources in my area that humans misuse or overuse? What will this mean in the long run? 
Once you hone in on one specific subject connected to conservation of nature, you could make one image of the subject the focus point of your poster. You could also do comparisons of areas where nature has been conserved, and areas where nature has been destroyed, to create a strong visual.