I was using Nikon digital camera, and recently bought a Canon 550D. I am totally new to SLR.
I am not familiar with SLR terms: Aperture, focal length and ISO, and their relation to one another.
What consideration should I take during
This is a very broad question, and I'm tempted to suggest that you buy and read Scott Kelby's Digital Photography book series. It's not the most technical series, but it gives you a nice introduction and tips for taking photos.
As for the more concrete question:
ISO was (is) the sensitivity of the film in a film SLR. In digital SLRs it's the analogue and digital amplification in the sensor. It's a number that describes how much light the film/sensor need to correctly expose an image. A low ISO will require more light. If you increase the ISO you can have a smaller aperture and shorter shutter time, and still get a correct exposure.
Aperture is the 'size of the hole' in the lens. This controls how much light that comes through the lens. A large aperture (denoted with a small f-number) will let more light through the lens. The aperture is also responsible for changing the amount of bokeh in an image. Larger aperture means you can use lower ISO and shorter shutter times.
Focal length translates into field of view for a given camera. A long focal length will give a narrow field of view, while a short focal length will give a wide field of view. A short focal length will be similar to 'zooming out' on a compact camera, i.e. you can capture more of a scene.
One important variable you forgot to ask about is shutter speed. This is the length of time of the exposure. A fast shutter speed (e.g. 1/250th of a second) will 'freeze' the picture, while a longer (1/5) will give you motion blur in the image. By increasing the shutter time, you can use a smaller aperture or a lower ISO setting.
To get the correct exposure you need to balance ISO, aperture and shutter speed. Most cameras will do this automatically, but with a SLR you can normally also have manual control over one or more of these variables. This gives you more control and "artistic" freedom.
Summary
All these effects are balanced:
Faster shutter speed - darker picture
Smaller aperture = larger f number = darker picture
Smaller ISO = camera sensitivity = darker picture
Less light = darker picture.
Focal length is not usually directly considered as it is automatically allowed for in the f number.
A typical day time scene requires about f/8 at 1/100th second at 100 ISO for correct exposure. Values are adjusted above or below these settings as light varies.
Many modern cameras can manage most or all of thes parameters for you. As you get used to their affects they become much less challenging to deal with.
Behind the scenes:
Firstly we'll deal with what an EV is:
One measure of light level is an EV or "exposure value".
1 EV is about twice as bright as bright moonlight.
Changing EV by 1 changes light level by a factor of 2, so
2 EV is 2 x as bright as 1 EV
3 EV is 4 x as bright as 1 EV
4 EV is 8 x as bright as 1 EV
So brightness ratio between two brightness levels X and Y
= 2 ^ (EVx - EVy)
where 2 ^ means two to the power of.
eg 2^3 = 2 x 2 x 2 = 8,
2^5 = 2 x 2 x 2 x 2 x 2 = 32
Now the relationship to cameras:
A brightness of 1 EV is achieved at aperture of F(1) at 1 second exposure with 100 ISO film or camera sensitivity.
Light into camera changes linearly with exposure time or inversely with fractions of a second of exposure. eg 0.5 second exposure allows 2 x as much light as 0.25 second exposure.
1/4 second exposure allows half as muchlight as 1/2 second exposure.
Light input changes with 1/f^2 where f is aperture f number.
This is because the f number relates to the diameter of a lens but area varies as diameter squared.
An f number is often written as eg f8 or f(8) or f/8 but is properly f1:8 or f1/8
SO for correct exposure
EV = 100/ISO x Shutter_speed x f^2
EV = brightness
Shutter_speed in seconds
f is aperture in 'stops'
ISO is camera sensitvity
Wikipedia f number says (also below)
Focal length does not enter the equation directly but is indirectly related by the fact that
f = focal_length / diameter
So focal_length = f x diameter
diameter is diameter of lens opening.
What the above says is that the light gathering power of a lens increases with diameter (actually proportional to diameter squared) and decreases linearly with increasing focal length. This result is actually reasonably intuitive if you draw a picrure of a lens which is pointed at an evenly illuminated surface. Lens frontal area directly affects light input. Focal length will affect the angle that the lens system "sees" and the area decreases as focal length decreases.
In practice you work with ISO, shutter speed and aperture to control exposure for a given brightness or EV level and focal length is taken care of for exposure purposes. Focal length has an extremely importnt effect on several inage attributes, but that's a subject for another question.
Wikipedia f number says:
An oldie but a goodie - you won't see many f0.95 lenses!
Photo: 1960's vintage Canon 7 with Leica M39 lens mount and 50mm f1:0.95 lens!
Here's a great website with an interactive simulation that will help you learn how these factors interplay when determining the exposure, depth of field, etc., of your photograph: http://camerasim.com/camera-simulator/
(Disclaimer: I've no relationship to the site, I just think it's pretty cool)
