"M" is for Manual and that means that you must use the camera's light meter and Manually set the correct aperture (aka f-stop) and shutter speed based on the ISO you've selected. These 3 elements - shutter speed, aperture and ISO make up what's called the Exposure Triangle.
ISO - A measurement of the sensitivity to light of a light sensitive surface, either film or digital sensor. A low ISO (50, 100) is very insensitive and requires a lot of light. A high ISO (200, 400, 800, 1600) is more sensitive and requires less light. A low ISO will give the best image quality while a high ISO can allow us to make pictures in lower light situations.
Aperture - An opening formed by the movable blades of the diaphragm inside the lens. It controls how much light is admitted by the lens. The smaller the f-number (f1.4, f2) the LARGER the opening and the more light admitted. The larger the f-number (f11, f16) the SMALLER the opening and the less light admitted.
Shutter Speed - The shutter speed controls how long the light admitted by the f-stop we've chosen is allowed to expose our film or digital sensor based on the ISO we're using. At f16 and ISO 100 our shutter speed will be slower than it will be at ISO 100 and f8. If we use ISO 400 our shutter speed at f16 will be faster that it was at ISO 100 and f16.
The shutter speed is also used to prevent blur when photographing sports or other action. The faster the shutter speed the less possibility of experiencing motion blur.
The Owner's Manual for your 60D has a section on shooting in Manual which you need to read. In fact, time spent actually reading & studying the Owner's Manual will be time well spent. When you have no idea what you are doing "... messing around with the ISO, shutter speed, etc. ..." expect totally messed up results.
The 3 elements of the Exposure Triangle are inter-related and interdependent. Changing one requires changing at least one of the other 2. The venerable "Sunny 16 Rule" demonstrates the relationship of shutter speed, ISO and aperture. It states: "On a sunny day set your aperture to f16 and your shutter speed to 1/ISO." So if its a sunny day and we're using ISO 100 here is what an exposure chart would look like.
ISO 100
f16 @ 1/100 sec.
f11 @ 1/200 sec.
f8 @ 1/400 sec.
f5.6 @ 1/800 sec.
f4 @ 1/1600 sec.
f2.8 @ 1/3200 sec.
f2 @ 1/6400 sec.
Remember what was said about the function of the aperture? The smaller the number the more light admitted and the larger the number the less light admitted. Remember what was said about the shutter speed being dependent on the f-stop and ISO? The more light admitted by the lens the faster the shutter speed at a given ISO.
As the above chart shows at f16 very little light is being admitted so our shutter speed is a relatively slow 1/100 sec. When we "open up" our lens to f11 the opening gets bigger and our shutter speed increases (gets faster) to compensate. By the time we've "opened up" to f4 our shutter speed is more than fast enough to stop action.
These sites will help you learn how to use your 60D:
http://www.ehow.com/how_8018449_use-eos-60d-dlsr-camera.html
http://dojoklo.wordpress.com/2010/09/16/canon-60d-users-guide-and-tutorial
Spend time at these sites if you really want to learn about photography:
http://www.digital-photography-school.com
http://www.kamerasimulator.se/eng/?page_id=2
http://www.illustratedphotography.com/photography-tips/basic
As you've probably begun to realize there is much, much more is involved in achieving good results than just buying a DSLR and expecting it to magically make you into a good photographer. Always remember this: "It isn't the camera its the photographer."
Historical footnote: The "Sunny 16 Rule" was invented back when few cameras had built-in light meters and even fewer amateur photographers owned hand-held light meters. Film used for prints had more exposure latitude so if the exposure was off by 1/2 stop or even 1 stop the amateur photographer would still get acceptable results. Today "Sunny 16" is useful to show the Exposure Triangle relationships.