Hello,
I have my textures in a format like .png or . jpg or any other normal format for pictures. What must I do to have these files in the orbiter .tex format?
The format Orbiter uses for texture files is .dds
You can use the dxtex utility included with Orbiter to convert .bmp images to .dds, or you can use a program like gimp or photoshop with a .dds plugin.