Let's see... we used to have a "How to write fanfiction thread" up here, I think... but I can't find it now. Well, I'll just help you the old-fashioned way, then!
Alright... I'm assuming you want to write a Pokemon fanfic, judging from the fact that you didn't mention another series you wanted to write about when you posted. (It's fine if you want to do another series; I'm just guessing at your intentions.) First of all, don't feel like you have to follow the storyline of the games or the shows in any way. The whole point is for you to have your own take on the Pokemon world, so it's usually best to deviate a bit from the standard idea. Many fics on the forum aren't about training at all, in fact. I remember one that was, at least at the start, about a girl who helped on an Arcanine ranch. There are also others that incorporate other elements with the training aspect of it all. It could have to do with a personal mission, saving the world, or even committing crimes. (Not overly violent/graphic stuff, though.) Just try to come up with an original concept and follow it.
Also, it's always a good idea to look through the chapters as you write them and edit each one before you post it. It can be very difficult to read unedited work sometimes, especially if you know that a single glance would help the writer to correct dozens of typos and such to make the work clearer.
Don't feel the need to rush, either. I see too many writers fall into the trap of posting a chapter a day (or more...) and not understanding why their chapters aren't getting as many good reviews as the ones that writers spend much more time on. Take your time; relax. Let the writing flow on its own. If you hurry too much, you'll burn yourself out in the end. After all, you don't start a marathon with a sprint.
Finally, just have fun with it. You'll probably have a people say they like your work, and you may have some who give you pointers on things to improve. (A few of us here are notorious for that, so don't be shocked if one or more of us gives you some advice.) And remember, we're all friends here. If we criticize you, it's not to be mean. It's so you can get even better at writing. So enjoy your time here, and have fun writing fanfiction! Don't forget to read, as well!