You need to earn 4 badges in your starting region to be able to travel to other regions. After that, you can go to the port city of your region to travel to other regions :

- Kanto : Vermillion City (dock at south area of the city)
- Johto : Olivine City ( south building of pokemon center)
- Hoenn : Slateport City (north building of pokemon center)
- Sinnoh : Pastoria City (south-east area of the city)
- Unova : Castelia City (2nd dock to the left from pokemon center)

If you want to get back to your previous region after you start your next region, you do have 2 options :
- If you have 1 badge or less on the new region, you can go back to the first town and talk to the mom to get back to your first region.
- If you already have 2 badges or more on the new region, you need to go to the port city to get back to your first region.

Note this will happen when you move to a new region :
- All items except HMs and key items will go with you.
- When you're visiting a new region for the first time, your current pokemion will be sent to PC box and you will get a new starter based on the region you chose.You can get your old pokemon back as you reach a PC and use them to play on your new region as it matches to your new region's level cap.

You can check level cap lists here :
https://pastelink.net/PokeMMOLevelCaps