On-camera flash is VERY unflattering.

If you have a good bit of light coming in the window, you might be able
to use it, possibly with one or more reflectors. You can get "foam core"
very cheap and cut it to size. Just make sure the reflector is larger
than subject you want to illuminate.[1,2,3]

You might try a home-made bounce (just a small piece of white cardboard
in front of the flash) or a Lumiquest[4] bounce for on-camera flash to
bounce the light off the ceiling.

You might also try finding older used flash units off E-bay. The
strobist website[3] is a good source of info on what can be done with
old cheap flash units. It's primarily geared toward Nikon, but there are
a lot of Canon users with tips there also.

Next up would be the DIY Home Depot studio lighting kit. Exact
recommendations vary. A thorough Google search can find good info on
getting setup with a very cheap, but good quality setup.[5]

Further up the price scale would be some purpose made Tungstun
continuous lights.[6]

Then you get up to the strobe lights. You can get a small strobe from
AlienBees[7] at very affordable price, and it will change your life.

I use a Nikon D70 with a couple of Nikon SB800 Flashes and a couple of
AlienBees AB800 Strobes. Both setups are useful, and I'm learning all
kinds of neat ways to use them. But I'm finding it very hard to stop
buying new lighting equipment, modifiers, etc.

Good Luck,
Randy.

1. <http://www.photoflexlightingschool.com/>
   <http://www.lastolite.com/>
2. <http://studiolighting.net/> (also subscribe to the podcast; there
are a lot of good podcasts on camera equip & technique, lighting,
portrait, wedding, business, etc.)
3. <http://www.strobist.blogspot.com/>
4. <http://lumiquest.com/>
5. <http://www.google.com/search?q=home%20depot%20studio%20light>
6. Eg. <http://photoflex.com/>
7. <http://alienbees.com/>