I do not own pokemon whatsoever, if I did I would have made this into the game.

First you need to make it just like the game is with EXP, levels, the bag, storage, popup menu for moves and items, etc. And it doesn't matter what pokemon you are as long as you're not a legendary. And if you pick one that's not an option in the original game it has to be fully evolved, your partner however has to be on its second evolution if you pick a non starter pokemon. Also gender does not matter, like you can be a male Eevee or a female Riolu.

Then you have the option to go to the guild for a short time to gain information about Grovyle but you leave when you figure out Dusknoir being from the furture (when your guildmate says he just showed up out of no where). You do have the option to tell your new partner about what's really going on but if you tell him or her the partner has to go with you. Or you can just try and find Grovyle but you'll have to leave your partner behind or tell him or her about why you are here.

You won't be able to tell any other pokemon about why you are taking the Time Gears, like the Lake Trio. Its your choice if you get found and dragged back into the future by Dusknoir.