Whats the best economical lens to use for indoor photograph with

The rebel Xti is old school technology with poor low light capabilities. You will need a lens that lets in a lot of light but since you are using a 10 year old Camera I doubt you will be spending $1500 on a lens. If you are photographing People, You need a focal length from 70mm on up. This will eliminate the flattening of the facial features caused by too wide of an angle lens. i.e. 28mm or 35mm. However if you are photographing rooms or interiors the opposite is necessary, you want a 18mm, or even a 10mm-22mm zoom. The Xti came with an 18-55mm But it's not wide enough for room shots. Canon makes the 10-22mm and you should fine it used for around $400. The 55-200 would be fine for people shots at the longer zoom range. 100mm and further out. But it is not a bright lens. If you can look for a T3i, or the new T5, They have 4x the low light capability, for under $500.

I recommend a 50mm f/1.2 lens. Cannon also makes a f/1.0 but I'm sure that's quite expensive. The f/1.2 is a very large aperture which allows plenty of light in for indoor photography. While there is no zoom capability, this lens is often called what-you-see-is-what-you-get. The focal length is pretty close to the human eye, so you'll need to move if you want to change your perspective. The low f number can also make some interesting photos with very narrow depth of field.

If you are new to photography in general and especially SLR photography I would suggest holding off on buying any lenses besides the 18-55mm.

The reason I suggest not buying extra lenses just yet is coz you need to get to know your camera and if you are new to SLR photography then you need to get your fundamentals right. Sure the 400D will let you shoot in full auto mode but you will really not learn much doing that and certainly not be making the most of your camera in that process and that will extend to your new lenses too.

Get used the the control, the 18-55 is a great little cheap lens with the pop up flash it will work great for most family pics. The 55-200mm is a new "cheap" lens, its been marketed as letting you go beyond 55mm thats why it starts at 55mm goes to 200mm.

Hope this helps you...