confusion about thai visas. transit or holiday?

ive just bought and paid for my first rtw trip.... ;-)

after searching through which visas i have to get, i found the thai one abit confusing.

many websites say that i can just get a 30 day visa stamp (free) and all i have to do is catch the flight, fill in forms on plane and bring a couple of passport photos of myself.........

another website says that you have to have proof of travel out of thailand eg, flight booked out so they definatley know your gonna leave.

my problem is that im planning on traveling thru thailand and flying out of singapore. Obviously this is possible as many people do it, but i dont want to get on a plane to bangkok and be refused entry as i havent got a flight leaving the country and the wrong kind of visa

if anyone could help id be very.v.v gratefull

cheers