Statistics Coursework

My hypothesis is: unemployment is greater in women aged 18-24 in Acton than it is in men aged 18-24 in Hackney.
I have the data I need and the next thing I need to do is "you are expected to show that you are able to use a sampling technique to reduce the number of data from your data collection to a more manageable number"
the best sampling technique to use would be a stratified sample as it would get me higher marks.
This may sound simple but it's really got me confused. I've got lots of data so do I have to sample them all? It also says "data collection n=100+, sample size n=25 to 40". I have a rough idea what that means but I don't know how to do it. Could someone explain what I have to do in different words that would be easier for me to understand please?
thankyou