Well, the first thing I'm going to suggest is... learn GML. Seriously. I learned purely from the GM help file, my only prior game making experience being that from RPG Maker 2000. It will take a while to get used to, but it is so much easier and fast in the long run than using the D&D system. But I'll try my best to help you with what you've got so far.
To set an enemy's health:
Open your enemy object [object4 in your project], and make sure the event tree for "Create is being shown. Then, from the D&D tabs, choose "Control". You should see the heading "Questions", "Other", "Code," and "Variables". What you want to do is define a variable to store the enemy's Maximum HP and its Current HP. So, under the [Variables] heading there are 3 icons, a square, an octogan, and a tan colored square. The first square defines a variable, so drag and drop it into your event.
Now you should see a window titled, "Set Variable." It is asking what to apply the variable to, the variable's name, the variable's value, and whether you are adding or subtracting from the variables current value (relative).
Leave the Applies to option checked on "Self." This means that the variable will forever be associated with this object and will be referred to as "object4.[variable]". In the variable box, enter: "enemyMaxHP" (Important note! Variable names can only contain characters A-Z, a-z, 0-9, and underscores "_"). Set the value to whatever you want your enemy's max hp to be (we'll use 10 for now). Leave the [relative] box unchecked. Now click OK. Congratulations! You've created your first variable. It's name is "object4.enemyMaxHP" and it holds the value of 10.
Now create another variable. Leave the Applies to option on self, name it "enemyCurHP", and set the value to "enemy4.enemyMaxHP", and leave the relative box unchecked. So, now you see, the value does not have to be a set of numbers. You can set one variable equal to another. In this case we are giving the enemy is maximum starting HP.
Now the reason we created two variables is so that we have one we can change and one we can reference. We want to change the enemy's current hp, but what if he were healed and had no maximum HP? He could become infinitely strong and unbeatable to the player. So we make another varialble that we can compare it to. We make the statement, "If the enemy's current HP goes above his maximum HP, we should set it back to his maximum HP."
Variable are probably the most important aspect of any programming used in games and you should become really familiar with them. Their uses are not limited purely to what I described either. Then can store all sorts of information such as text, lists, colors, etc.
I hope this helped and wasn't too confusing. I tried to make it as easy to understand as I could, but I may not be the best at explaining things.
