If you're asking such questions, I reckon you're not a photography enthusiast who demands fine-grained manual control over advanced settings. As a noob myself who have only played with cheap compact point-and-shoot cameras on auto, I can only offer some general advice.
Am looking for a slim portable camera that shoot hd photo,
Get a compact camera. All compact cameras in recent years are at least 16 megapixels or 20 megapixels, way more than HD. So no worries.
All are able to shoot 720p videos at the minimum, and some can do 1080p videos.
Then you should be looking at compact camera.
Mirrorless, compact, superzoom, what the different?
Mirrorless is a compact alternative to DSLR cameras. Allow you to change your lens but also more expensive than compact and superzoom cameras. For serious photography enthusiasts who do not mind spending more money for flexibility to "reconfigure" their cameras with different lenses and good image quality. I believe to get started with mirrorless camera would cost from $800 to $1000+. But if you're just starting out with photography, I think no point jumping for a mirrorless camera, and wait till you realise the need for it. Question is do you think you would want to switch camera lenses?
A compact camera is one you can slot in your pocket. Easily portable. Good for quick casual shots on the move. Optical zoom mostly between 5x to 10x. Relatively cheap, and advanced models exist to allow manual control over more settings. If you're planning to shoot only in auto mode all/most of the time, a compact camera is sufficient.
Superzoom/long zoom cameras are cameras with 20x to 60x zoom. Good for zooming in on something from long distance, e.g. wildlife photography, scenery shots, architectural shots. But they're usually bridge cameras with DSLR size, so it is not what you're looking for. Major difference compared to DSLR is that bridge cameras cannot change their lens.
Also the specs - what to take note?
I strongly recommend you pick a compact camera with Optical Image Stabilization (OIS) to prevent blurry shots due to shaky hands, unless you want to carry a tripod everywhere.
Note that many cheap cameras (< $200) only come with Digital Image Stabilization (DIS) and it's not effective and/or lead to poor images.
things like Maximum ISO, etc
To me as a noob, it doesn't matter how high the ISO can go. It can stop at 1600, 3200 or even higher. But the higher the ISO, the more noisy -- and lower quality -- your image becomes.
Some cameras can claim to support extremely high ISO (like 6400), but it only means the potential for worse image is there.
Ideally if you can shoot at ISO 100, the better it is, depending on lighting condition. Generally more light leads to lower ISO and better image quality e.g. outdoor shot on a sunny day. Less light leads to higher ISO and lower image quality e.g. indoor shot or night time shot.
The 3 factors ISO, aperture and shutter speed are interlinked. You can read for better understanding how they affect each other:
Understanding ISO, Shutter Speed and Aperture - A Beginner's Guide
with so many camera, what would you recommend? something that affordable around 500 or below
I shall leave it to the pros around here to recommend. But generally going with a Canon or Nikon compact camera wouldn't be too far wrong. Every brand has its own fan base.
You should read reviews on a camera model you're interested in before buying and look at some sample images taken by it if available. Trust your own eyes.