There are lots of places you can start. Some people like to change Pokemon and overworld sprites first. Some prefer to work on the trainers. Personally, I prefer to start on the maps.
To change Littleroot Town, you'll need to find and download a program called AdvanceMap. It's on this forum. You can use it to change a map's terrain and name.
Two things to remember:
- The terrain blocks don't determine whether or not you can walk/surf/etc. on something. To change that, you have to edit "movement permissions". (So in other words, placing a wall won't block a space. You have to set that space's movement permission to 1. Walkable ground is C and surfable water is 4.)
- You can't add map names, and some map names are reused. So if you rename Littleroot Town, then every other map with that name will also be renamed.
Good luck on your hack.