I wrote out a LONG in-depth answer to this... and then the power went out as I was proof reading it...
So the reader's digest version is this:
Like the others said you need to anneal. Heat it to dull red, then cool, either air or water doesn't matter. You need to stretch a ring around the center where the bottom will turn into the side. Use a big hammer with a rounded face and beat it into a sand bag. Whenever you sense the metal becoming harder to work, you need to anneal again.
Once you have that area stretched down to where you have a semi-flat area for the bottom, you need to planish it to even it out. Using a broad faced hammer with very little crown LIGHTLY hit the metal over a solid backing. A good chunk of steel that has been thoroughly smoothed will do.
Now, the sides... Look up "Tuck shinking" on youtube. You ought to be pretty good at it by the time you finish this project

because you have a LOT of shrinking to do.
That is the quick version. Your success will depend greatly on how thick the material is. If you have more questions, I'd be glad to help.
Other than that, Good Luck!:rockin: