How can I fuse galvanized sheet metal with a circular bottom together so that it is water tight?
Soldering is the usual method, but without metal-forming equipment I'd guess you don't have, that's not going to be a likely (or affordable) road to success for someone with no experience, for any value of large. 

Buy a galvanised washtub or trashcan is the direct, simple route to a watertight circular metal "pot" over a wide range of sizes. Move into "stock watering tanks" for a larger size, though most of those are more oval. Without exhaustive shopping around:

17 gallons, $30:


31 gallons, $19, with lid:


This one is 477 gallons, circular, and costs $219. 


Now I've gone and started to think that would make a nice country-style hot tub, and the price is so right...