Pretty much any bag will do to start you off and see how you get on with it.
The more important factor is decent bag gloves, ie ones that won't leave you sweating, won't rip and fail, and will protect and support your hands.
Definitely avoid any gloves that come included with the bag, or any cheap vinyl ones!
Probably worth watching a few videos on bag work, too, to avoid ******* your wrists up...