Consult your university or department's dissertation guidelines. Check to see if they have any instructions on acknowledgments. In most cases, they will only specify where in the work the acknowledgments section should come.
Make a list of all the people or organizations who have supported you while you were working on your dissertation. This can include financial support, technical or research assistance, or even just advice.
Write a short section thanking people who helped with your dissertation. This can be as short as one paragraph, but can be longer if you have more people or groups you need to thank. Begin with groups who supported you financially, such as bodies awarding research grants. Each of these should have a sentence to itself. Next, thank those that provided research support, such as access to laboratories or documents. Lastly, thank those who helped with advice or guidance. If you wish to thank your advisor, place him or her in a prominent position such as the end of the paragraph.