You want to make your own Operating System... and you are trying to learn the language..hmm..
Any program or maching can do multi-tasking ( or doing more than one task at a time )
To make a program multi-task.. just code the language to do so...every program I know of can multi-task AND for the most part does.
A computer can perform the tasks or in this case "multiple tasks" required by the program.
The ONLY bad thing would be that the computer might not be fast enough to handle the tasks given by the program.
Relax though.. unless your computer was made in 1995 you can probably do some serious milti-tasking in your code.
OK SO.. You are 15.. and you are NOT too young to learn Assembly Language.
I know of people that are 8 years of age with degrees in many areas...
I think you should start a whole lot smaller than what you are attempting to attempt...AND I really don't think you are redy to make your own Operating System.
I really don't see any point for you making your own "OS".. although I can't answer a question invloving why a programmer does what he/she does...Programmers have been known to do crazy things..lol
I recommend you learning another language..for example.. QBasic. This will give you a feel for programming languages.
If you already know QBasic.. which I hope you do.. ( if you don't know QBasic then..I don't understand how you could possibly want to learn how to create your own "OS".) then try coding more complex programs.. ones that may perform functions within your computers.. and keep editing it and learning from it.
If you are 15 years of age.. havn't had college training..and trying to learn this on your own.. umm.. GOOD LUCK!
If you need any questions answered I may be able to help you with them... any questions other than coding an "OS". - general programming though, I can help with.