go for it, it will be the trip of your life!
check the ads on the internet
type "crew wanted" in google and search from there.
floatplan.com is great and free, so is 7knots.com
the place to be is Panama or mexico, panama being the best by far. you can easily get to know the skippers by helping out on the canal passage from colon to PC before going for the whole pacific crossing. ask at the yacht clubs and marinas, visit the anchorages, post your own ads.
the time to leave is about late march/ april, few boats will leave for french polynesia before that as it is still cyclone season in the south pacific until june. if you are there at the right time, there are usually more boats looking for crew than crew looking for boat so you can pick the boat you prefer, something that doesn't happen often in the rest of the world.
it is a big trip so you need to make sure the boat and the skipper are safe, the best way to do that is to ask around to the other boats. if the skipper you want to go with is popular and people are talking well about him you are safe, if everybody think he is an ass then maybe you should reconsider if you want to spend a lot of time with him on a small boat.
it takes one month more or less to reach the marquesas, another week or two for tahiti, after that it is mostly a couple weeks between island groups all the way to australia. it is about 8000 nautical miles to queensland, it is a really long trip. most cruisers spend a year or two doing it. few go non stop and those usually are professional on deliveries.
you need to have money saved to enter polynesia, the authorities will ask you to show a proof of fund, to show you can fly away if you get off the boat. the captain is responsible for the crew but you don't want to be an ass and put that burden on your skipper you need the money in your bank account, about 2000 dollars more or less.
it is normal to share the costs, few skippers will take you for free as they don't need your work or your help, they just will take you for company and to make sailing easier. usually it costs between 5 and 20 dollars a day to share the costs and the food, depending on the type of boat and the number of crew.
cargo ships do not take crew like you , they only hire professionals. the time when travelers could cross ocean on them in exchange for work is revolute.
hope this help
check my own website so you get an idea of what crewing on a sail boat is like: