Hi Adrian,
I'm also using 450D and a happy owner of it for more than a year now. I personally shoot in Manual mode + Manual Focus so I'll share some of my exprience with you
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When you are shooting in Manual mode, you are taking over the camera in control of exposure with your desired setting adjustments, and it is totally different from Auto Mode where the camera will auto adjust everything. Managing exposure in Manual mode is by adjusting shutter speed and aperture size (f-stops) to control the Depth of Field (DOF), ISO levels, white balance and flash exposure compensation (if you use flash). These settings will then be converted into Exposure when combined together.
When to know you can shoot? It's hard to figure out in the beginning but you'll get used to it. First, you do the settings which you think will be suitable for the light conditions. Next, do a half shutter and see the metering bar at the bottom of the viewfinder. You should see metering bars from -2 to 0 to +2. These are the values to tell you if the metering exposure from the center of the viewfinder is underexposed, well-controlled or overexposed. For me, it is underexposed at -2 to -1, well-controlled at -2/3 to +1/3 and overexposed at +1/3 to +2. Once you get the exposure right, look through your viewfinder and start turning the focusing ring of the lens (set your lens to Manual Focus first). When the AF beam light starts blinking (red blink) at your desired AF focusing spot, it indicates that the area which the spot is in, is in focus. Now, double check your exposure bar and make sure that the exposure is good and press a full shutter.
You may have a question in mind. Why the AF spots in the viewfinder will lights up even you are using Manual Focus? It is simple. Basically, it is there to assist the photographer when he/she is not sure when the focus is right. Older SLRs like the ones I borrowed from my cousin many years back, don't even have these features.
Next question you may ask is how the aperture, shutter speed etc affects exposure. Basically, below is what I understand personally.
Aperture (f/stops): The smaller f/number means the wider the aperture blades open where it allows more light to go into the sensor. Downside is the thin DOF which means the focused area is smaller when the aperture is wider aka smaller f/stops. How wider can you go and how narrow can you go depends on the lens you mount onto the camera. If the lens is a f/1.2 lens, you can go all the way to widest at f/1.2, likewise if it is a zoomlens which is capable of a f/3.5 to f/5.6, it basically means from the shortest focus range it can go to f/3.5 widest and will increase as the lens zoom out, where at its longest zoomed focus range, it is only capable of f/5.6.
Shutter speed: The lower the shutter speed, the more light is allowed to go into the sensor before the shutter closes. Downside is camera shakes where you one will tend to have some small movements before the shutter closes and results in the whole shot burred out. Tripod is usually needed for slow shutter speed aka long exposures. Slowest is usually 30 secs or BULB mode when you will control how long you want the shutter to open. Usually, a cable release is recommended for BULD mode to prevent camera shakes. Fastest shutter is dependent on camera body.
ISO levels: Mainly to help boost the light receptance by the camera sensor where you will be able to shoot with more ease when the light is low. At higher ISOs, the camera's sensor will boost it capabilities to light up the picture for you, allowing you to use higher shutter speed and higher f/stops, without the need of flash (especially in places where flash shooting is not allowed). Downside is noisy picture as the higher the ISO, the more grainy the picture is. Especially in the darker portions of the picture where light level is low. Though said, some likes the grainy touch and use the shots for Black & White (B/W) presentations.
White balance: To balance the colors under different conditions. E.g. Tungsten W/B for shooting in places with orange light hues so that the camera can used the settings to control the temperture of the light colors back to normal white lightings. Sometimes, W/B is intentionally set differently to the enviroment to create a different mood for the picture.
Flash exposure compensation: Mainly for flash photography. Likewise the numbering of exposure level of -2 to +2, this is a custom function for you to adjust the flash power. If you want more flash power to overcome the strong sunlight behind your subject so that your subject is not underexposed, do the flash exposure increment and vice versa for not overexposing your subject to give a natural picture. Though said, this feature can be very subjective at times as sometimes a few trial and test shots are needed before getting things right.
I know it's quite hard to understand all at once. It is still the best to shoot more and learn from every shot you took. Never waste your shutter counts and learn nothing from it. Though said, till date I'm still learning alot of things from fellow photographers, outings and the forum so I'm never afraid of trying and learning at the same time. To me, if I never fall, I'll never know where I made mistakes in. If I don't accept teachings, good/bad comments and good/bad critiques on my shots, I'll never know how to correct to make the next shot better or express the next shot better. Though said, sometimes, there are some bad comments may be pure rant or very subjective towards a different style that you want, can be taken with a pinch of salt.
Hope that the above helps
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