You have selected an interesting question. As with any work of literature, there can be any number of identifiable themes. The Giver is no different in inviting a wide number of different interpretations that might focus on topics such as what it is to be human, the relationship between pain and pleasure, and the role of the individual. For me, however, more important than these themes is the role of memory.
What is interesting about this book is that the author was inspired to write it after visiting her aging father who had lost his long term memory. This led her to realise that without memory there is an absence of pain. If you forget experiences that caused pain in your past life it is as if the pain did not exist. Jonas is part of a community that at some stage decided to eradicate pain by eradicating memory. This allows them to create the perfect utopian society, where members are free to live in happiness, but also are free of urges to engage in any activities that will bring unhappiness to them and the community of which they are a part. As Jonas discovers, however, memory (and the suffering that often accompanies it) is necessary to truly live - the half lives that his friends and family live as depicted in the novel make this truly clear. The unescapable message of this novel is that pain and suffering is part of the human condition and makes life the rich tapestry of experiences that it is. If we forsake our ability to suffer by eradicating memory, we are not truly human.
Remember that a theme is a big/overarching idea in a piece of literature. For example, "love" is a theme in Romeo and Juliet. "Justice", "prejudice", and "innocence" are all themes in To Kill a Mockingbird. Can you think of one big idea that is present in The Giver?
One you have identified a broad theme, you will create a specific theme by answering the second part of your question. What does The Giver teach you about your big idea? You shoudl be able to actually answer this question in one neat sentence, although you would need additional sentences to support your answer.
When you answer the third part of your question, you'll need to bring in something from the real world that connects to your sentence (if you believe the theme is relevant in the real world). Or, you will need to prove that the theme in The Giver does not connect to the real world at all.
The most important theme in Lois Lowery's novel "The Giver" is that Life is not really being lived if there are no emotions, feelings, and differences among people. The author uses the idea of memories being taken away from people. Memories are the key to the things that make humans real. They contain all the things that humans have to draw from that enable them to respond to different situations. Without feelings and emotions people in the society were completely controlled by a government. They made no choices for themselves. They had no memories to draw from if a situation occurred beyond the restrictions of their society. However, they knew that someone had to have the knowledge and so the Receiver contained all the knowledge within him or her.
The theme is that a society needs memorys for greater knowledge in the world. for example: what would be the point in having knowledge of something if you didnt know what it was