Assuming you have picked a Saturday to visit, follow the crowds out of the station past the chocolate-box mews houses (look out for George Orwell's blue plaque) and you're straight into the antiques section of the market, which goes from Chepstow Villas to Elgin Crescent - you can download a useful map here: http://www.portobelloroad.co.uk/getting_there.asp
Obviously the market is a tourist attraction but Portobello Road is fundamentally a working market for Londoners and a great Saturday afternoon out; I haven't managed a trip there yet without leaving laden down with fabulous finds. Go, enjoy.
Yours, vintage clad,
Girl About Town xx