Once you have chosen one you need to think of potential clients in that market, take teenage lifestyle for example - the market will be teenagers. You need to interview a client and ask them what the problems are that they have which relates to your chosen area. For teenage lifestyle this could be broken down into the home, at school, social etc.
Under each category write a list of problems in that area e.g it being hard to concentrate on school work at home or something like that. Make sure you ask people so you are getting real problems and not just making them up
Having a real problem to solve is important because you can test your project later on and see if it solves it, if it doesnt your client can give you feedback on how to alter it and you will gain marks for this.