@aardvark250 Your camera, known here in the USA as Rebel T4i, has what is know as a crop sensor. That means it gives lenses a more telephoto look than a camera with a full frame sensor, like the 5D or 6D or 1DX series. So for telephoto that is a good thing.
Professional telephoto lenses can cost over ten thousand dollars and weigh enough to require a special tripod head, but I think it is safe to assume you want something affordable and portable. There are lots of options, but as a general rule the more expensive a lens is and the shorter the zoom range is, the better the quality.
I am a semi-professional photographer and follow a Canon photographer's forum and most people agree that the sharpest long telephoto that is easily portable and affordable (for Canon) is the Canon 100-400 f/4.5-5.6L IS ii. There is a ZooChat thread on it here:
the perfect zoo lens
Affordable is a relative term and this is still a pricey lens, at around two thousand US dollars (though it has a good instant rebate at the moment). If you can afford it that is the way to go, but if not there are other options. Canon makes a similar 70-300 f4-5.6L IS that is smaller and a bit less expensive, but still amazingly sharp. If you do not know, Canon lenses that are professional have L in the title. For telephotos, they are also white instead of black. I think this lens is just over one thousand USA.
If that is still too expensive, you can get a non-L Canon 70-300 f4.5-5.6 for a few hundred dollars. You can also get either a Sigma or Tamron 100-400 for several hundred. These are both newly released lenses and very sharp.
As someone stated above, Sigma and Tamron also make somewhat new 150-600 lenses. These are bigger and heavier and their sharpness gets mixed reviews (some people say they are great and some say they are poor quality in the 450-600 range). There is a ZooChat thread on those as well:
150mm - 600mm zoom lens
That is a lot of information but I hope it helps. If you can afford the 100-400 f4.5-5.6L ii (make sure it is the version ii), then my advice is get it and look no further.