How close do I have to be to a subject for it to be in focus at f/1.8?

[astroNikon]

Yes thank you none of this is new to me... I know the basics and I have a tripod and wireless remote, "I already realize" That I need flash. my problem is I can't afford lighting equipment at them moment. I'd LOVE to but Its just not possible for me to spend $200+ on flash. So for now, I have to deal with what I've got.

Press that little lightening bolt on the side of the viewfinder in the front area .above the FN button to popup the flash
==> Nikon D3000 Controls

 

[Kawaracer]

For example

This picture is taken with a D700 and a Nikkor 80-200mm f2.8 lens at 155mm shutterspeed 1/500. The focus is on the guy with the hat at just about 15 meters away, this gives a DOF of 1.58m and about 75cm in front 83cm behind the focus distance. I could have taken any head even the trees to focus on. The distance doesn't matter, but if I took the trees everything else would be out of focus and that woudn't give a nice picture.
So technically any distance the lens is able to focus in is possible, but you have to keep in mind if you get a nice picture with a only a smal part in focus, when you take a landscape with one tree on 100m where you focus on with a aperture of f1.8 the OOF would be to to dominating and it woudn't be a nice pic. For a porttrait it is the same thing, dou you want that much of OOF?