I'm coming to US in October from Aust. and need to purchase cell phone for travel particularly in the New Englands.

My current Ozzie cell does not work in the States.

I have heard you can pick up cells at Wallmart.

I would appreciate all help in what to buy and where to buy. I will have one day in San Fran. prior to going to East Coast and need some store in San Fran. area.

Prefer getting one that has no ongoing cost or the cheapest option.

I really have no idea what to ask or look for.
Many thanks