The way you are going to be able to blur the background on an image is as has been said a wide open aperture. The faster the lens you have the better chance you are going to have to do that. If money is tight you might try something like a 50mm 2.8 lens which can be had in the $100 neighborhood and with ast lenses the sky is the limit from there. You can slightly blur the background with f/4 but from there it gets difficult.
There should be a mode on your camera called creative auto it's next to full auto. In creative auto there should be a little slider and it will ask you if you want your background more or less blurry. There are limitations though by the lenses that you have. The higher the aperature the more blur you're going to get, for example at f/4 you're going to get a slightly out of focus background. If you had an f/1.2 lens though you could throw the background completely out of focus.
My suggestion would be to use the 28-135 at 135 and shoot in Aperature Priority (which on a Canon I think it's Av on the rotator knob) and shoot at the highest aperature or lowest number.
Another suggestion would be to look into getting a fast prime like a 50mm f/1.8. If you shoot at f/1.8 on that lens you're going to be getting a nice out of focus background. One of those can be had new for $100 from here or used for $70 here.
Thats a whole lotta camera for a beginner ( 7D ). I wish I started out with a 7D...(stares off dreamily ) haha.
If you find that you are predominantly shooting people/portraits. I would recommend the 50mm F/1.4USM. I think mine was like $350. It does not zoom at all so you have to walk closer or away to adjust your distance, but it will blur the background very nicely. It will also perform much better in darker lighting. If you are on a budget you can also go with the $99 F/1.8 50mm that was already mentioned. It has a cheaper build quality but if you aren't looking for an investment, just a quick and cheap method, go with that.
You can also blur the background in photoshop but it gets a little tricky to make it look natural like a lense does.
Seek out the camera manual and read it - then read it again - then read again with the camera and play with it Try putting the camera into aperture priority mode (AV on the dial) and play around with using different apertures and see how they affect your shots as well as how they influence your shutter speed. (ps my camera manual lived in the camera bag with the camera for the first half a year to year that I had my camera - best place since you can check and try things when shooting rather than having to come home nad find the manual)
I would also strongly recomend the book Understanding Exposure by Bryan Peterson because whilst its still aimed atbeginners it will go into more depth and detail as well as giving more examples on how you can use aperture, shutter speed and ISO to control and manipulate your photos.
The 50mm f1.8 is a popular lens and is often recomended because its very cheap and yet has surprisingly very good optical quality. Its build is rather poor though (very plasticy) and there are more durable options on the market but they cost a lot more. That is why you see the 50mm often recomended - 50mm was also the standard lens supplied with older generation cameras.
My own advice is that before you start looking at more epxensive options like the 85mm get more practice with your existing setup as well as more understanding. The choice of which lens is the best to get is very hard to impossible to make if you don't have a decent working understanding. It won't take long to get nor is it that hard, but I feel that it is an important step.
For what you are asking in your first post your kit lens can still do the job you are wanting and do it well, but you have to learn to control it - if not then even with the right gear you will still get results you don't desire.