I agree with Bertine re: paying for the Early Bird service, and Spareroom.co.uk is probably your best bet for finding a good flatshare.
What kind of commute are you hoping for? Travelling from Hammersmith to Canary Wharf seems like it could be a bit of a pain, so if I were you I might be tempted to consider places in east London, or maybe even north London to get the young professional vibe you seek.
For example, taking the Overground from somewhere around Highbury&Islington/Canonbury/Shoreditch and changing at Canada Water for the Jubilee line could be alright. Highbury in particular is quite popular amongst young professionals, and has a nice range of restaurants, pubs and bars for socialising. I just travelled through Shoreditch tonight at about 8:30, and it was definitely buzzing -- but could be a bit noisy at night. You could also consider near Angel/Old street station and take the Northern line down and change at London Bridge. All of these places are still quite central, so it shouldn't take
too long to get your bearings, and may be a little more convenient travel-wise (the district line can be a bit temperamental...).
I like
rentonomy to get a general (visual) idea of where places are in relation to one another, and also to compare rents/crime rates/council tax. If you click on a particular area, it pulls up a profile with different tabs describing transport options, leisure facilities, the general population (displayed as a pie chart with various age ranges).
If you're considering different areas,
Ludlow Thompson Area Guides can be helpful. For some of the places I mentioned (Highbury, Angel) it doesn't say much, but I'd say they match that which is captured in the ones for Canonbury or Islington.
Have a look and see what piques your interests most of the places already mentioned by yourself/Bertine, and consider these in line with what your commute would look like-- heck, even try it out during the morning/evening rush between when you arrive and when you start work, to get a better idea of the daily bump and grind.
But I do think you should be able to find a houseshare in the first two weeks -- just do make sure you see the place and meet the flatmates first, to get an idea of whether you could actually live with these people, and what sort of state the flat is kept in.
This website gives a good starting point for things to consider/ask when you meet potential flatmates.