Although my photos aren't great, I would say the best advice is just take a BUNCH of pictures. Try everything, play with settings, see how far you can push things. Try taking the same picture many many times with different settings to see what it does. Once you start getting the technical stuff down a bit more, you mind is more free to think about the shot you want to get and you aren't worried about how you are going to get it. Reading the "rules" of photography, the tips and tricks (the rule of 1/3s, etc.) is not a bad idea, just make sure you then break all the rules and do what works for you. Now that I have just said to ignore the rules, one of the great sites for learning about lighting is
http://strobist.blogspot.com/
I would say if you are just starting, don't go to the full manual modes quite yet, I would try futzting around with either the aperture or shutter speed priority modes (Av or Tv modes), that way if you know if want very shallow depth of field, all you do is you set your camera to Av mode, set the f stop you want, and the camera will set the shutter speed for you. It's nice as sort of a semi manual mode. That way, you can set the parts you care about, and watch to see what the camera does to set the exposure, and you can start getting a feel for the amount of light you need to properly expose for the settings you want.
As for a lens for the XTi, it heavily depends on the style of photography that suits you. Which lenses do you have now, and what do you feel that they are lacking? One of the great walk around lenses for the XTi, and one that 1.6 croppers are loving is the EF-s 17-55 f/2.8 IS (B&H has it for $949 right now) really fast lens which also boasts image stabilization, and a fantastic zoom range, which is the equivalent to a 28-85 on a full frame camera, but again it depends on what you are after. I use a 24-70mm on a full frame 5D, and 24mm is pretty wide, but if you are seeking really wide angle, 17mm on a 1.6 crop camera isn't going to cut it, and you might want to look at the EF-s 10-22mm f/3.5-4.5. A bit over a stop slower on the long end (so half the about of light reaches the sensor = twice the amount of time the shutter has to be open) but it is a fantastic ultra wide.