I'm going to disagree with some of the posts made here.
First off, I think an entry level camera is a great place to start off. As you'll probably hear later on: Lenses over bodies. Even an older entry level like the XS or XSi has more than enough features for a beginner, and with the leftover money you can buy lenses, flashes, etc. to give you more versatility in your photographs.
Between the XSi and T1i: The T1i has video and +3MP. Whether this matters to you is up to your own preferences, but as I said up there, I'd rather just take the XSi and build up my lens collection instead.
It should also be noted that equipment =/= image quality. Having the right equipment does help making good photographs easier, but skill matters so much more here. If you're worried about having "bad equipment" that puts out subpar photographs, don't. Skill will play a much bigger factor in that than your equipment, period.
Between Canon and Nikon, you can't really go wrong either way. I'd just go for the brand with ergonomics that suit you the most. It's a personal choice, not something that you can figure out online, so go to your local brick and mortar and try out these cameras in your hands yourself. Just a note: Nikon's entry level cameras (D3000, D31000, D5000, D5100) lack an in-body autofocus motor, so Nikon's AF lenses won't be able to autofocus on these bodies. As a new shooter, it probably won't matter too much to you as you've no legacy glass and it's easy to find AF-S lenses (which can autofocus on these bodies), but something to consider. Canon doesn't have this concern.
As for my personal opinion, I've used pretty much all the Canon and Nikon crop bodies, and I'd say that they're all great cameras, and you really can't go wrong with any of them as your first DSLR.
Finally, I just want to reiterate: Read up on manual photography and practice, practice, practice! Dream Killer and I have written two photography guides that are stickied on here, and are a good place to start reading up on photography basics.