OK, then to the basics.

First you need to keep in mind that the engine always cuts of partial numbers, so any mathematical operation has to have the division or decimal multiplication last to keep the error from growing.

That said, it is a relatively simple series of control variable commands - which is an extremely versatile command.

It is also good to reserve some variables for temporary operations, never storing a value there long-term.

control variable HPcost = actorXY's HP (can be selected under game data, the line that defaults to map id but has a lot more behind it)

control variable HPcost *= 0.10 (multiply by 0.10, this would give 10% of the original value, and this is where you have to know percentage mathematics - as ozubon said above you need to know what to multiply the value with to get the correct other value)

change HP: actorXY (or possible "entire party" for your game), decrease, variable HPcost

That is a series of three event commands that in combination do what you want.

If you have problems with percentage mathematics you can ask us here for help, but I strongly advice to brush up on your schoolbooks in such a case as a "survival" type game will require the developer to know his mathematics for balancing.

I also suggest you follow the link to the "starting point" in my signature. It lists a lot of tutorials and specifically a tutorial game called "variables gude" that you should play before going on with game development