For dating service photos, I would recommend capturing a few different styles: full-frame face, half body shot, and full body shot.
Start with the portrait; this will probably be her primary photo. I would recommend using your T4i kit lens, EF-S 18-135mm IS set at 50mm (80mm full-frame/35mm equivalent) or greater. The long focal length gives you two attributes that are desired for portraits: less facial distortion and shorter depth of field (blurred back/foreground or bokeh).
The problem you have using a short focal length is that the nose and chin appear larger than they actually are; 50mm or greater focal length solves that problem. However, this may move you outdoors due to the fact that you'll have to be far from your subject.
The second, short depth of field, puts everything except your subject out of focus; the bokeh effect. I can think of three factors that effect depth of field, but first, let me describe it (DoF). Depth of field is the distance before and after the point of focus that is within the range of acceptable focus; Objects outside this appear blurred and will no longer distract from the subject: her face. The strongest of the three on DoF is aperture or f-stop: the smaller, the shorter DoF, the more bokeh effect and better for portraits. Your kit lens has a low f-stop of from f3.5 to f5.6; this range is spread throughout zoom of focal length from 18-135mm. At 50mm, the lowest (a.k.a. fastest) is f5.0. Note also that the longer your focal length, the shorter your DoF. Distance to the subject also decreases DoF; the closer, the shorter. Use the DoF Preview button to check bokeh; It's located on the camera near the lens mount on the bottom left side under the lens release button.
To keep things simpler, I'd recommend setting ISO to Auto (button is in front of the mode dial).
On the T4i there are two or three good ways to achieve favorable portraits, and this has to do with which mode you'll choose to shoot in (the mode dial on upper-right). If you have knowledge of photography, you could choose Av mode (aperture preference or Aperture Value). You can set f-stop (Av) by rotating the dial behind the shutter; left is lower f-stop, more blur. The Portrait mode does this automatically for you. Third, the CA (Creative Auto) mode has a blur scale: again left is lower/more blur.
Notes about these modes: There are settings that favor portraits. In Av mode, you can set the Picture Style to Portrait; this will soften the image (press the down arrow to control this). It's also important to get the White Balance right. It's important to match the light source so that white objects appear white; that way skin tones look accurate. If you've ever seen an orangeish indoor snapshot, that's due to bad white balance. The T4i has three types of White Balance settings: Auto White Balance (AWB), specific type, and custom. The T4i's AWB is very good at detecting the correct WB, but after reviewing the images, if skin tones look off, then try setting it to existing light source. Press the up arrow/WB button, left/right arrows to choose between Daylight, Shade, Cloudy, Tungsten (incandescent light bulbs), Fluorescent, or Custom. After reviewing, if skin tones still look off, resort to Custom. I shot a wedding in a church with the nightmare of halogen, florescent, and incandescent lighting; digital camera w/CWB was perfect, 35mm film camera images were unusable.
The T4i's method for setting Custom White Balance is a little more involved than for their point-n-shoot cameras (e.g. G12); It's a four-step process (pg. 135 of Instruction Manual). First, take a picture of a white object. I keep a white card in my camera bag just for this purpose. Make sure to angle the card so that the most light hits it without reflecting off it. This is a reference photo you can keep on your memory card, and since the resolution isn't important, you should shoot it in the lowest S3 image size; just remember to set it back to L before taking your portrait. Second, select Custom White Balance on #2 Menu. Third, import the photo of the white card by choosing that photo. Fourth, press the up arrow/WB button and choose Custom. After shooting, when your light source changes, remember to set WB back to AWB. The reason Canon chose using reference photos is that it gives you unlimited Custom White Balance settings. The Canon G12 only has two CWB settings, but are easier to set.
In Portrait and CA modes, things are much simpler. You can set the Ambience to Warm; usually set to Standard. To set this, press the <Q> button; it'll be the top setting on the LCD display. Press Set, then use arrows to move to Warm, then press Set again.
Now onto lighting. I recommend existing lighting over flash. If you must use flash, try an external and angle it 45° to bounce off the ceiling. If using the internal flash, you may end up with red-eye despite the T4i's ability to reduce it. Use ImageBrowser EX or ZoomBrowser EX software on your computer to remove red-eye.
I recommend shooting outside. If you can shoot on an overcast day, do it; it acts like a natural diffuser, avoiding sharp facial shadows created by the nose. It also reduces squinting, crow's feet, small pupils, etc. It also gives you room to backup and use a longer focal length (zoom in). Dating sites tend to want to see full body shots, outdoor hobbies (bicycling, gardening, softball, etc.). Use Sports mode for action shots.
Play with eye contact. Looking down the lens gives a good sense of personal contact. Looking off camera can add a flavor of candidness and intrigue. Looking within frame at a subject (a prop, bubble gum, jewelry, etc.) or other person (a child, meter-maid, farmer's marked clerk, etc.) can tell the beginning of a story the viewer may be interested in hearing the rest of. It's important to get a picture of your subject laughing; So, for those shots, get her out of her comfort zone. Have someone off camera through an apple at her unexpectedly to catch; use Sports mode with Continuous shooting. Shoot as many candid shots as possible; Unfortunately, unlike Canon's PowerShot models, there's no way for the T4i to shoot silently (the mirror flips up). You'll have to ask her to ignore you. Have her in different outfits, maybe revealing body parts (legs, cleavage, belly, etc.) within her comfort zone. If she's got nice eyes, try obscuring parts of the face with a scarf, her hands, part of a building, in order to emphasize them. Try framing her face using branches, have her look through a window, even using her hands. Switch up framing: if you predominately shoot in landscape, try portrait; and visa versa. Motion everyone but her: Using a tripod, ask her to stand still, set shutter speed slow (1/5 second), then ask others to move around her, turn their heads, wave arms, etc.; so that she's the only one unblurred. Try some night shots. Night Portrait mode used with a tripod with lights in the background is nice (flash will fire more than once, so tell her not to move until after the second flash). Or use Handheld Night Scene mode; tell her to stand still for one second. It'll take four rapid photos and merge them into one.
Most importantly, gain her trust and patience. Explain to her that in order for these photos to come out spectacular, you both are going to have to work equally as hard. That's why models get paid the big bucks. You both must put aside the time to get it right; snapshots you'll get in a snap; photographs take time.
Let me know if this was helpful, or if you have any other questions.
-=- PalaDolphinhttp://www.PalaDolphin.comMy equipment are:
Canon Rebel T4i / EOS 650D
lens: EF-S 18-135mm 1:3.5-5.6 IS STM
lens: EF-S 55-250mm 1:4-5.6 IS II
lens: Tamron SP 60mm F/2 Di II 1:1 Macro
Canon Speedlite 380EX flash
Canon PowerShot G12
Canon AE-1 Programmable w/55mm, 200mm, & 24mm lenses
bag: Lowepro® SlingShot 202AW