The reason they are called supporting ideas is because they “support” the topic sentences. As a rule the supporting ideas come after the topic sentences. They so-to-say “support” the topic sentences by telling the reader why he considers that the topic sentences are true.
Nearly everything can be used as a supporting idea in the essay. There is one important thing about them, however, they are to be related to the topic sentences and they are to support them in some way. Here are several issues, which can be used for supporting ideas:
• Someone’s personal experience;
• Something an author has heard or red about;
• Some logical argument;
• Some statistical data;
• Any factual information;
• The opinion of an expert.

Anything of the mentioned above can be used as supporting ideas. Therefore, it is also necessary to point out that the great majority of students use their or their friend’s / relative’s personal experience or something they have heard about. The reason of it is probably because it is much easier to express these things on paper.

So, let us return to our topic sentences and write several supporting ideas for them.
Here are the steps, involved in the process of writing supporting ideas:

 Step 1. Let us have a look at our first topic sentence. Here what we have so far:

 First of all, there exists too much violence in the nowadays world already.

Step 2. You should add transitional phrases to the beginning of the sentences. These phrases are like: moreover, in addition, on the other hand, etc.

Step 3. Take a look at your topic sentences and add some information, taken from the newspapers, television, someone’s personal experience in order to support them.
Now, after we have our first topic sentence in front of us, it is possible to use something that we have heard or about which we have read for our supporting ideas. Let us consider some newspapers articles. And, finally, we get the paragraph, which looks like the following one:

 First of all, there exists too much violence in the nowadays world already. There are lots of stories in newspapers about violence, about people who are shot because of something or even accidently. Moreover, police officers play main parts in some of these stories. In case if guns were not involved in some specific situations, these dramatic accidents could not have happened. Therefore, on the other hand there are very few stories about how someone’s life was saved with the help of a weapon.

After we are over with the first topic sentences, look at the second one. And while writing our supporting ideas, it is possible to take someone’s personal experience into consideration. And here is one way we can write the second paragraph:

 In addition to the points, mentioned above, in the majority of cases police officers do not need guns. For example, my friend’s father has been a police officer for more than 25 years, and there was no time he had to use his gun. In fact, he considers that the great majority of people, who break the law, are not armed in any way. Thus, there is often simply no need for police officers to carry guns with them.

And, finally, our third topic sentence comes. It is possible to support with some ideas, taken from a story, which you heard from someone. And your third paragraph may look like this:

  To finish up with, any kind of violence is a bad thing. Someone has told me that a car thief was caught by the policemen, and after they caught him, they simply shoot him, although he was not armed at all, he just said there was a gun with him.

And, finally, here we are! These are our supporting ideas ready. All of them help the reader understand why each of out topic sentences is true.