Scratch

I suppose you meant in Scratch, so: It's as simple as creating a variable for health, and keeping track of it.

When the game starts, you reset it to a certain value. When the player touches an enemy, it's life is decreased. If the player's health reaches 0, game's over.

when gf clicked
set [Health v] to [100]
forever
    wait until <touching [enemy v]?>
    change [Health v] by [-10]
    if <(Health) < [1]>
        broadcast [game over v] //you decide what to do here.
    end
    wait until <not <touching [enemy v]?>> //so we don't lose too much health. optional.
end

Use a variable for health.