The process is called UV mapping (or UV unwrapping sometimes) and is not that easy to explain, but here are a few outline steps:
1. Select your model.
2. Get->Property->Texture Projection->XY,YZ,XZ (any one of these, doesn't matter)
3. Select polygons on your model which you wish to UV map
4. Open the textureeditor, (Alt+7)
5. Choose a subprojection type (planar, cylindrical, spherical etc.) which suits your polygon selection best
Repeat steps 3 to 5 until you have mapped your entire model.
There is a LOT more to getting good at UV mapping. I've been doing this for a year and a half in XSI and two years in 3DMax, and I still sometimes make bad calls and mistakes.
For planetary textures, I suggest you use a cylindrical UV subprojection for the central part of your planet and then map the poles with a planar subrojection. That way you avoid texture distortion at the poles which is pretty much unavoidable with a spherical map (even though there are some ways you can pre-distort the texture to fit your spherical mapping better, but I won't go into that and its not that effective either).
