The power switch has an "On" position and a "Very On" position, with a white line against it leading up towards the thumbwheel on the back. In this "Very On" position the thumbwheel is enabled and you can easily apply exposure compensation up to +/- 2 stops in 1/3 stop increments. I'm not sure that it has any effect in the basic zone modes, but in P, Av and Tv modes you can quickly change the exposure a little by twiddling the wheel. In the first "On" position the wheel is disabled so that you do not accidentally move the wheel and end up with your exposures all wrong without noticing.
If you shoot manual then you need the switch to be in the "Very On" position in order to enabled the thumbwheel and allow you to adjust the aperture - so I guess you're already using it that way.
HTP is Highlight Tone Priority and is a new feature introduced first in the 1D III and included in the 40D. It adjusts the camera's internal processing to try to protect highlight detail without you having to do anything special each time you meter and shoot. When you use HTP there is a danger of slightly increased noise in the shadow areas, so it is not good to have it enabled all the time, only when you need it.
If you are shooting manual and blowing highlights then obviously your exposure is too high. I hope you understand that when metering it is not necessarily the objective to have a centred meter reading. If you are shooting manual then you are in full control. The meter is telling you how bright the metered area is relative to 18% (or 12%) grey. It is not going to tell you whether your exposure settings are correct. That is up to you to figure out, using your judgement and experience.
If the scene before you is generally quite bright then your exposure should be set so that the meter needle is pointing a little above the centre. I can't tell you by how much as that depends on the scene, how much brighter it is than middle grey and what your creative intent is for the scene. If the scene is generally a bit dark, and you want to preserve that look, then you will need an exposure that places the meter a little to the left of centre. Again, by just how much depends on the exact scene and your creative intent.
You also have metering modes to consider - evaluative, centre weighted average, partial and spot. Depending upon which you choose, you will have more or less sensitivity to where exactly in the scene you point the camera. Spot will cause wild swings as you point the camera at lighter areas and darker areas in the scene. Evaluative and CWA will be steadier but still move around a bit. Where is the sun, or the brightest part of the sky, when you are metering, inrelation to the overall composition?
Without seeing example pictures and the EXIF data with them it is hard to give any exact answers.
The bottom line is that if you are getting overexposed shots then you need to reduce your exposure settings - either reduce ISO, increase shutter speed or close the aperture. If you're shooting manual then changing any one of those things is simple. So just do it. The big point to remember is that the meter needle is not going to tell you when your exposure is correct - it just tells you how bright your exposure is relative to middle grey.
Think of it like a car speedo. You're driving along a road and coming up to a bend. The speedo tells you how fast you are going but it makes no judgement on whether that speed is safe or not for the corner. Say the speed limit is 60mph but this is a tight bend and better taken at around 30mph. You can probably take the corner OK at 40mph, in a good car, (let's assume you have traction control and dynamic stability or something (pretend that's HTP on the 40D), but any faster and you will not make it (overexposed). You could go at 20mph and play safe but what a dull ride (definitely underexposed)! 27mph, 33mph, whatever speed you go at, none is right or wrong. The meter will tell you your speed, not whether it is the right speed. The right speed is your judgement call.
By the way, if 90% of your scene is relatively dark and then there is an incredible bright element in it - the sun - then you will struggle severely to set an exposure that looks nice and captures everything perfectly. The sun will almost certainly blow. You don't need to see detail in the sun itself, so that is not the most important thing to protect, in terms of exposure. But you do need an exposure that preserves the colours in the sky immediately around the sun. Sometimes deliberate underexposure can enrich those colours and enhance the scene. You need to get creative.
Here's one of my sunset scenes. This was shot to jpeg in Program mode, with 0 exposure compensation. I used spot metering to meter the sky near the sun but not including the sun. I did not want the brilliance of the sun to throw off my exposure. The centre of the sun is blown, but I do not think it matters. Final exposure settings were - 100 ISO, f/14, 1/500. The picture is unedited, just resized.
Even though I used P mode, I didn't need to apply any exposure compensation because I chose to meter in a way that gave the result I wanted.
If you shoot manual then you apply "exposure compensation" manually, by setting your exposure to place the needle above or below the centre of the meter. If you shoot in P, Av or Tv modes then you set exposure compensation using the thumbwheel to constantly adjust the exposure to always be a bit above or a bit below what the meter would like. Say you're shooting in a snow scene in daylight. The bright white snow will throw the meter off. It will think the scene is too bright - white snow is brighter than middle grey. If you don't take steps to adjust the camera your snow will turn out middle grey. It does not know it is pointing at snow. So you dial in some exposure compensation - probably around +2 and your snow will stay white. If you were shooting in manual mode in the snow then you would want to set a manual exposure that was around +2 stops above the centre of the needle.