How can you create an entirely new tileset?
You can just make a new one and save it in the stage folder, you can copy the pxt file from another tileset if you want to save time assigning type tiles, but otherwise the spritesheet of the tileset must be as follows, assuming vanilla circumstances;
1. The tileset may not be larger than 256×256 pixels, you may go smaller if you wish (which can open up to a really cool trick which is not important here).
2. Tiles will be 16x16 pixels, this means that you can have a maximum of 256 tiles in one tileset.
3. The filename must start with 'Prt', otherwise the filename does not matter.
4. The file must be in .bmp format, renamed to .pbm if you haven't done the pbm>bmp conversion yet (if you use booster's lab, this should be done automatically).
If your tileset is done just save it in the data/stage folder of your mod, and it should be usable as a tileset. The only thing left is a .pxt file but booster's lab should make one for you if you try to select your new tileset, you can then just right click a tile in the editor to bring up a screen to select the collision type of the tile (which also include water and spikes). That should be about it unless I'm mistaken.