My friends,I need advice for Digital Camera,help me.

These may help you out. But usually you want decent(4+)megapixels, quick shutter speed and optical zoom if you can get it. Also make sure your camera has a rechargeable battery and good battery life.

http://www.newegg.com/ProductSort/CategoryIntelligence.asp?Subcategory=12
http://www.pcworld.com/article/125645-1/article.html

Since you are ready to spend 500 dollors I would like to suggest you a sony brand. I think sony is best brand for Digital Cameras.
I think you will like Sony Cybershot DSC-N2. This is the best camera I have ever came through. I have this one and I'm telling you with the personal experience.
It is 10.1 MP camera with alot of different features. It is touch screen camera. It has the largest possible screen to view the photos you have taken. We can view the photos directly in the T.V as well. It has slideshow feature. And the battery is quite long lasting. And the most important part is it has internal memory as well.
Hope you will really like it.

We just brought A720IS because we read good reviews. It had way too many settings and I could not figure out. My pictures ended up yellowish.

For Digital Camera, you should try Canon. Loads of photographers prefer their digital quality. Sony is not too bad, as they do have the anti-shake feature, meaning if your hands are not stable, the picture you take is not blur. I'd say, you should try it around to see what you are comfortable with, and see what qualities you want from the camera by importance.=)