It sounds like what you need to write is an essay - an expression and support of your personal opinion regarding the topic of assisted suicide. Therefore, the first thing you need to do is determine what you believe regarding this topic and find some information to support your point of view.
If you feel assisted suicide is justified in some situations, why do you feel this way? What circumstances make this an acceptable action? Are there conditions or guidelines that should be in place to control the use of methods for assisting persons to commit suicide? If you are against assisted suicide, why do you consider this an unacceptable alternative? What reasons do you see that prevent this course of action from ever being an acceptable possibility?
Once you have determined your position and found some supportive information for the reasoning behind your position, you are ready to write your essay. Your thesis statement is a brief initial presentation of your point of view and the reasons why you feel as you do. In the body of your essay, more fully present each reason individually and use your supportive information to justify your stance. Conclude by summarizing the points you have made and reemphasizing the position you took in your thesis statement as being the correct view, in your opinion.