*magically appears from a cloud of green, glittery smoke* 'Allo! I'm y'er Writing Fairy. Let's get started!
Start with an attractive character! No, I don't mean look-wise, but a character either y'er attracted to in relatability, their thought process and emotions (the soul or "character" of y'er character). Next, figure out how they'd act, speak, do the daily motions. With that information, create an appealing backstory that ye find interesting, yet realistic. The job right now is to keep ye interested and invested in what y'er making. Because, if ye don't even find them interesting, why would y'er readers? Even if ye never fully give out their backstory in the book- things like they had an aunt who was a magical sorcerer who helped the innocent, or perhaps had a cousin who cursed a village, it still models and shapes the character they are at that moment and ye can still use that information in a later book.
With y'er main character born and ready f'er action, ye'll need side characters to help him along in his dangerous quest to save the Purple Raccoon of Wisdom- or just finish high school. Now, ye won't need to go as deep with these characters as y'er main man (or woman) of action, but it never hurts to delve in there and make them special. Why is Mr. Peterson so strict as a principle? Perhaps he lost a student in an accident caused by his neglect of being the teacher and not the friend? Giving side characters some depth may just help out in the long run and may even turn out to be more of a main character later on.
The universe is as big as y'er imagination, so be creative with y'er world! Even if this isn't some high-fantasy or sci-fi adventure, making more of a world than South Lake High and Timmy's bedroom will help get readers involved with y'er story. The world is bigger than the two locations ye hang out the most, right? So don't f'erget the details! Make a map of y'er world if that'll help ye. If y'er using Earth as home base of operations, design a town instead of using one already made- it'll give ye more range to do things.
Main character, ensemble characters, setting, now to the nitty gritty of writing- the plot! The plot is basically the backstory of y'er setting. What has happened to y'er world to make it the way it is today? Are their super villains running amuck? Perhaps y'er story is animal based and the world is completely filled with garbage of the past. Where'd the garbage come from? Why is new garbage appearing and who is dumping it there f'er y'er furry friends to deal with? A plot is not simply the present- 'tis also the past and future. Plan ahead, and plan behind! Ye'll never know when a good prequel will jump out at ye.
I hope m'advice will help ye along the way of y'er writing travels. One last bit of advice I can give ye is to find a writing community to share y'er work with and receive support from! This can be around where ye live or on the internet. I use Wattpad f'er publishing books f'er people to read. 'Tis free and ye don't 'ave to worry about people stealing y'er novel ideas due to copyright options built in f'er ye to pick the best copyright label f'er y'er stories.
Best of luck! If ye ever do happen to join Wattpad, feel free to look me up @Dreaming_Fantasies and send me a hello! I'd be happy to help ye further in y'er endeavors as a writer, so don't hesitate to ask f'er assistance. Anyhoo, I can't wait to see what ye come up with in y'er stories.