if you are a newb to any of this, i'd suggest looking at
this this this and/or
this.
also, there is some random information on modeling and textures in
my weapon shop thread.
you can change the heightmap (landscape) by using multiple shades of black/white on a 256x256 PNG image; darker shades are lower, and lighter shades are higher. this image must then be saved into Lugaru>Data>Textures. (this is delicate, however, i would not recommend attempting this unless you have a lot of image editing experience.) OR you can get a program like terragen, and have it generate some perlin noise, then export the heightmap from there as PNG and put it in the Textures folder. (still delicate, but better success rate.)
Lugaru modders do not need to code at this time.
im not sure what you mean by "how textures are mapped" but when a model is created it must have a UV map, and this is where the texture is applied, with the correct colors to the right verticals then edited further in an image editor such as
GIMP.
to create a new map, you must enable debug mode; in the Lugaru>Data folder you will find "Config.txt" open this in a text editor, and change the "Debug" value to 1, save it in plain text format, and start lugaru. open any level in lugaru, and press 'Shift'+'M', this will activate the in-game map editor. for map editing instructions open the "Debug Readme.txt" file, inside are all the commands for debug mode and level manipulation.
i hope you asked your questions in [at least] slight awareness of the difficulty of moding games.

the information i've given you to look at will either be a rude awakening

or a steady guide
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