A great book for you, at your age, would be "Grimoire for the Apprentice Wizard" and "Companion of the Apprentice Wizard" by Oberon Zell. He teaches you how to respect Magick in all it's shapes and forms. Not only Magick, but all of the elements and the Universe itself.
https://www.amazon.com/Grimoire-Apprentice-Wizard-Oberon-Zell-Ravenheart/dp/1564147118 Oberon has a school of wizardry named the "Grey School of Wizardry" that you may want to look into as well.
http://oberonzell.com/ Magick is basically built around "focused will and intent". The same goes with "Prayer" and the same is with our practice here of Astral Projection and any other thing in life you wish to achieve. The rituals used are to help you can hone/lock on that focused will and intent into what you are attempting to manifest.
When you wish for money or some other thing. It doesn't just magically appear. By making that conscious intent, you are creating that new reality in your life. You may come across a new job opening or could also win some money in a lottery or something like that. But it's you that has to be "proactive" to get it to manifest. Even with the lottery. If you didn't buy the ticket, you wouldn't have won.
But, and this is a big but, you must always make sure to follow any wish or ritual with the words, "with harm to none, so mote it be so it is done". That way, nobody gets hurt by or during the manifestation of your wish or desire.
Now there is free will. So in the case of this girl you like. She has free will also. You can't force someone to like you. But by thinking of her, on some level she will get the message. Where it goes from there is up to her. Like I said, she has free will too.
Magick is neutral. The "black and white" comes from the intent of the practitioner. A so called practitioner of "white" magick can use what is considered black magick to actually achieve a positive outcome too.