The function of the thesis statement is to express the writer's position, the conclusions he/she has drawn, or the point he/she wishes to make regarding a certain topic Therefore, it really consists of two parts: topic and assertion. The assertion should be clear and contestable, as a position paper is written to convince the reader of the author's opinion (position) the thesis statement should not just express a fact.
Cannabis should be legalised.
Please note that you should not use 'I' when formulating the thesis statement, even though it expresses your personal opinion.
Of course, the thesis statement has to be defended in the body of the position paper in order to persuade the reader to accept it and therefore it is important for the writer to be able to find valid arguments to support the thesis statement (see body).