For a comprehensive answer, go to my post about writing and publishing here.
The best training is to read, read, read. If you're a bookworm, you already have a head start on everyone else who wants to publish their projects. Also, read not just the classics, but read what's being published today. Plot, pacing and characterization are much snappier in books being published today than, say, fifty years ago, thanks to competing media like YouTube channels, TV shows and movies. So if you want to be published the traditional way (with an agent and traditional publisher like Random House, Simon & Schuster, HarperCollins, Lee & Low, Candlewick, Chronicle, etc.), you have to keep abreast of the marketplace. That said, don't focus too much on the market, trends, what others are up to, etc. You want to keep laser focused on your projects and not let the noise distract you too much.
And one more thing: rejection is all part of the process. I promise, it'll make you stronger. The key is to let the rejection fuel you, rather than letting it break you. During the years when agents and publishers rejected my work, it would sting for a while, but then I became even more determined to make my next project better. And even when you're a published author, rejection is still part of the game. You never know whether your next manuscript will sell or not, so rejection is something all writers in every phase of their career must get used to (unless you're Stephen King or J.K. Rowling or...you get the picture).