Basic stuff, I guess. Can I somehow make a sun out of a light-source in XNA? I would like to be able to see the light, actually, and set it on the upper right corner like a sun.
It's possible to use a point light source for the sun, whether you are wanting to simulate what the sun looks like if you look at it, or the just light it shines.
TLDR; yes, but make it as far as you can away from the player and don't have any light falloff.
Essentially you can think of the sun as being a point light source that is infinitely far away. No the real sun isn't a point nor is it infinitely far away, but it's "close enough" you can think of it that way. The problem is, how do you make a point light source that is "infinitely far away"?
If the point light is too close, then shadows will not all be cast parallel. Image you had a fence with a gap between every board, and the sun was setting behind it. Every beam of light coming through (or board shadow) would be parallel. But if you had a closer point source, the shadows would spread out in a sort of fan, leading back to the point source.
Also if the point light is too close, and it is visible via some kind of lens flare or halo glow, the player will notice their position is different relative to it as they move. Imagine if the real sun were frozen in the sky on the western horizon for a moment. If you start walking south, it's going to basically stay in the same place in the sky unless you walk a very long distance. But if the sun (or your point light) were close up, then you would see it change position in the sky as you walked farther south.
Lastly, for all intents and purposes, the light from the sun doesn't fall off (get dimmer at a distance). Yes it does if you move millions of miles away from it, but if you go hundreds (or thousands) of miles nearer or farther away, it basically stays the same brightness as far as your eye can tell. So you need to make your point light source do the same - not have a falloff where it gets dimmer as you get farther away.