A relevant work experience can significantly strengthen both your university and job applications. Consider which types of firms operate within the sector that is of interest to you, research the organisations within each area and ensure you know whether they offer structured programmes for sixth form students. If they do: make sure you apply, making a note of the dates and deadlines. If not, you can always email the firm enquiring about opportunities for a work experience. Ensure that you tailor your email to increase the chances of a response.
If organisations in your industry of interest do not offer any experiences for your age group, think outside the box: which skills will I need to demonstrate to my future employer to be successful in securing a job in the future? What other types of organisations can I learn these transferable skills from?
Moreover, don't forget extracurricular activities. Ensure you join any relevant clubs at school and/or participate in competitions. Take advantage of networking events and speak to professionals within the industry. Always consider volunteering opportunities. 
Finally, although face to face experience tend to develop a broader skill sets, don't forget relevant online opportunities and courses, that will also be beneficial to you.