It'll probably depend how old and experienced you are.
I'm not a very experienced press tog but I'd expect you might be doing general grunt work, carrying bags, holding flashguns, holding brollies (the wet kind), being sent to get tea/coffee, uploading images, that kind of thing.
I guess it's the same for assisting any kind of tog in that they are all different, some will want you to be their slave, running around while they get the glory, some will want you to do everything so they get a rest and some will be quite willing to teach you everything they know and treat you like an equal.
Fingers crossed you either get chucked in at the deepend and asked to do it all or someone that enjoys teaching.