Okay,
LIGHTING:
5-in-1 reflector? Great. Fantastic buy. Try using the gold side in sunshine. It might help to have someone hold the reflector for you, so an assistant could be useful!
If you're shooting in mid afternoon sun, you'll be getting harsh shadows on faces most likely, as the sun is directly overhead. In this case a bit of fill flash could be useful (flash just used to fill in those shadows). A flash unit and off-camera flash cable would allow you to flash light your subjects from the sides, and not just dead on. You could also try using the onboard flash in conjunction with your reflector (i.e. bounce it off the reflector towards your subject, to diffuse it a bit, or fire it through the diffuser your reflector comes with).
However, natural light and reflector should be sufficient.
LENSES:
The 50mm 1.8 is a nice lens. Although I'd be inclined to go for the 35mm equivalent. For portraits a fantastic lens for your 3100 would be the Nikon 55-200mm DX G f/3.5-4.5. It gives FANTASTIC sharp images, is perfect for portraiture and won't cost you an arm and a leg. Alternatively you could also look at some 3rd party lenses.
SETTINGS:
Stick it on P mode and try using an exposure compensation of +0.3, which is my standard. Check the screen to see what you're getting, and adjust the exposure compensation until you feel you're getting good images. In general you want to be shooting with large apertures and fast shutter speeds (higher F numbers, smaller shutter speed numbers). You can get this in P mode by rotating the selector/control dial to the right (I believe).
ALSO!
Check your image by switching to the preview mode and zooming in on them to check details, focus. Don't be nervous. Take it slow and try and make sure your friend and girlfriend are relaxed. Smile, relax and talk to them. Try and get natural poses. If you can make them laugh, then great! You want them to try and ignore the camera as much as possible, nothing's worse than a picture of family members/friends etc. that is overly posed and awkward. Your friend and his girlfriend (I am assuming!) are not models. So don't make them pose more than necessary!
Good luck. And make sure you shoot more pictures than you want. You can always delete pictures (you shouldn't though, you should just keep the rejected images in a separate folder) but you can't easily go back and take more! If you like a particular composition or angle, take a few (or more) shots, move a bit and adjust the exposure compensation a bit between shots. That way, when it comes to reviewing your pictures you'll have a lot of options for that shot, you'll hopefully make sure the eyes are in focus and you'll have at least one with the right exposure. This will give you more choice.
Stop and think as well, before you shoot.
Remember....Eyes, eyes, eyes! Focus on them. Make them important parts of your photographs.
Good luck!
