If you wish to go back now, u can choose a starter then talk to Mom, she will send u back to Unova.
For pokemons, you will get to keep them but you can't use them unless you have them under your obedience level (check trainer card).
Obedience level allows you to use pokemons up to a certain level and is progressively increase with badges owned in that region. All regions have different obedience level limit except 0 badges = lv 20 and post E4 = lv 100.
You can keep items you own and use them except for key items, eg bicycle, HMs, watering can