Q:

How can I take selective color pictures with my Canon T1i? 

I want to select a certain color using my camera so only that will be recorded as color, with all the rest in gray- or b/w-scale.  Meaning, only one color and its shades appear colored, and rest is b/w. Something like the picture below:
http://blaberize.com/wp-content/uploads/2009/12/selective-color-wallpaper-city.png

A:

This is not done in-camera but in post processing.  If that structure was the only red thing in a frame then selective desaturation would have done the trick, else it is pretty much just selecting the structure and desaturating the rest. 

A:

My answer is going to be short, on this one:

Your camera can't do it, and
Even if it could, do it in post.  

Why?  Because you might wish later that you had the full-color image down the road.  Secondly, post-production will almost always give you a better result than the camera itself will.  There are many options that will do this in Photoshop, Paintshop pro, Gimp, etc.  

A:

I would take a normal picture and later process it in some software.  There is Tintii for example where the standalone version is free, so you can try it.  Here is a nice example taken from the Tintii photo stream on Flickr:

Then there is a "manual" approach possible - in GIMP for example.  I used this technique to create this picture.