How to choose an appropriate population sample for a uni research proposal?

As part of a university skills project, I am preparing a research proposal that should analyse drinking behaviour in a UK youth organisation (14-25 year-olds) by comparing it to the overall teenage drinking behaviour in the country (yet in the same age group). There are data available for the latter, yet it is not clear from the NHS report what particular data belong to the age I am considering. I am struggling to decide (calculate) an appropriate sample size for both of the groups. There are millions of 14-25 year old people in the UK and the organisation has more than 270,000 members. Surveying this many people is going to be too costly and not necessarily efficient. How can I tackle this? I hope you can help me with this. Thanks!