The more familiar you are with your camera and lens features and functions, the better your results will be.
Set your lens to 55 mm. At 55 mm it will have a maximum aperture of f/5.6. Select
A mode (Aperture Priority) and set A mode to f/5.6 (see page 76 of your D3100 users manual that is on the software disc).
At a focus point distance of 8 feet your Depth-Of-Field (DoF - the field of acceptably sharp focus) will be 1.4 feet deep. that 1.4 foot deep zone of focus will start 7.35 feet in front of your camera, and will end 8.78 feet from the camera.
If you move closer than 8 feet, the DoF will get shallower, and if you move further away the DoF will get even deeper.
The further from the background your subject is, the more blurred the background will be. If your model is leaning against a wall, that part of the wall inside that 1.4 foot deep DoF will also be in focus.
Note: The plane of acceptably sharp focus is essentiall parallel to the plane of the image sensor in your camera. If you tile the camera up or down, you tilt the plane of sharp focus too.
Here is an online DoF calculator I used -
Online Depth of Field Calculator
For making images of people in an urban environment, a primary concern needs to be light, light quality, and light angle.
Professional on-location shoots are usually done in the morning or in the evening (golden hours) to take advantage of a low Sun angle and a warmer sunlight color temperature.
Apparently you do not have a speedlight (hot shoe flash unit). But the built-in flash unit can be used for on the lens axis lighting. Unfortunately on the lens axis lighting tends to make people photos look flat, but used in manual mode with the power level turned down some it may be your only option. see pages 57 and 58 of your D3100 users manual that is on the software disc)
At the least having a reflector or 2 would be be helpful.
http://www.dg28.com/technique/why_we_use_lights.htm
You can get inexpensive white foamboard at office supply stores.
Or a reflector like this
Impact 7-in-1 Rectangular Reflector Disc (42 x 72")
The Sun would be your 'main' light. Reflectors are used to provide 'fill' lighting.
Unfortunatly, you can't put the Sun where you want it, and you can't make it brighter or darker.
Another key is to use spot metering mode to make sure your model is properly exposed. (see pages 79 - 81 of your D3100 users manual that is on the software disc)
You also want to be sure your camera auto focus is set to AF-S, or single focus, and that the auto focus area mode is set to single point AF (see pages 135, 68-70 of your D3100 users manual that is on the software disc).
The center auto focus point in your camera is the most accurate one because it is a cross-type focus point.. Use it to focus on your subject, and then recompose your framing.