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Pick a subject you enjoy that you know something about or are into as a hobby and build your site. It most likely will be rubbish but so what we all started somewhere and my first ever website still lives today although there is a good case for putting it out of its misery and its only ever earned me about £4 :-)
Earnings have improved somewhat since then and yours will to if you stick in there and believe you will make it - I can still remember making my first 5p commission and jumping round the room as it proved to me money could be made and its all I needed to fuel the next few years hard work.
You will need to devote some time in the intial period but you can often find that after the first 3-4 months of getting a new site from idea to completed you only need to spend a few minutes a day maintaining it. Depends on the site of course but I do personally have one that uses no PFP takes 20 mins a day to maintain and make £7000 - £8000 a month and 95% of that is profit.
My best advice is pick an area your into - get your basic tools from the net for free and jump in with both feet. Sooner you get started putting pen to paper and make some mistakes and wrong turns the sooner you will be grinning at the first cheque to come through the door.
Good luck in your efforts