Examples of bad thesis statements
The following thesis statements can not be acceptable:

Police officers work hard;
Guns help to protect police officers;
Police officers do not need guns.

As for the first of them, there is no author’s opinion about the problem, discussed. In addition, it does not inform the readers what the essay is going to be about. And, actually, it has no direct relation to the topic of the essay.
And although the second unacceptable thesis statement does reveal the topic of the essay, partly, of course, still, we do not see the author’s personal opinion, expressed in it.
Now, let us take a look at the third unacceptable thesis statement. There is no doubt that it may be true that police officers usually do not require any guns, still, no author’s opinion is expressed in this unsuccessful thesis statement. And the question of the essay is should police officers carry guns?

Step 2: Decide how strong your thesis statement should be
After you have taken a look at the examples of good and unacceptable thesis statements, you should choose the “strength” of your thesis statement. Let us consider the example of one of the successful thesis statements:

 Police officers should not carry guns with them.

There is no doubt that this is quite an acceptable thesis statement, although it can not be considered to be a great one. It is still possible to improve it by adding some expressive words or phrases, such as no or never. And here how our thesis statement will look like after we have improved it:

 No police officer should carry a gun with him.

It can also be something like this:

 No police officer should ever carry a gun with him.

In case if you are going to make your thesis statement a positive sentence, it can possibly be strengthened with such a word as always. And here come the good example:

 Police officers should always carry guns with them.

Therefore, it is quite possible to make already strong thesis statement even stronger. It can be done with the help of some specific phrases, which are usually used to emphasize the idea. To the most common of these phrases the following can be used: under the circumstances, at no time, without any exception, regardless of…, no matter, etc. And in this case our thesis statement can become something like:

At no time police officer should ever carry a gun with him.

It is also possible to use the phrases, mentioned above in order to provide the opposite opinion in the thesis statement. Then it may sound like this:

 Regardless of the situation, police officers should always carry guns with them.

And, in conclusion, it should be pointed out that the only thing which really matters in writing your TOEFL essay successfully is practice and practice again. You should also keep in mind that a thesis statement is really the most important sentence of your essay, and in case if you work hard, you will be rewarded by a higher TOEFL score.