seems pretty basic..
I recommend creating it without visuals first..
first, both the player as the enemy should have a maximum health and a current health variable, this will make it possible to show a bar with the remaining health..
I'm guessing each attack has a certain damage amount.. so when an attack is chosen this damage will be subtracted from the opponents current health.. offcourse depending on the action of the opponent..
so something like
player action is punch
-- opponents action is not defence
subtract 5 from opponent health
subtract 2 from player health