In order to prove this thesis by the Greek Thales, "All substances can be reduced to water." what would i have to do?! im clueless...

I doubt if you can prove Thales' thesis -- but these sites will give you good insight into his thesis.

http://science.howstuffworks.com/h2o5.htm

http://www.thefreelibrary.com/Earth:+a+watery+planet:+water+covers+70+percent+of+our+planet.+And+97...-a085465333

http://www.waterinfo.org/resources/water-facts