Stimpacks: Should probably be some in lockers or something in V15 but it might be emptry, could always go back to Shady Sands and buy some from a trader, I think Ian can sell you some. (not 100% sure though.)
Weapons: Go to Junktown if you have some supplies to sell then trade for a better weapon.
To initiate combat you need to press the combat button, this should activate the turn-based mode.
Otherwise you could left click on the weapon slot to start combat.