How to reduce the size of an image without reducing its clarity? For ex: if the image is 1000 pixels by Width and 750 pixels by height, how to resize the same to 300 x 250?

Usually I use the Image Resizer. It will probably be set on a ratio by default. I'm sure you can turn the ratio off and then set your OWN width and height limits.

Image>image size..and make sure the ratio adjusts automatically

to get from 1000x750 to 300x250 you can not be in the same ratio. Or am i wrong?

If the ratio isn't the same, you can create your OWN ratio, however it might not look as good, if you manually create your own ratio. It's "usually" best to use the default ratio levels in Photoshop.

If you do not stay within the same ratio, your image will be stretched in one direction or another.

However, if that is not a major concern for you, make sure to sharpen the image after resizing, to get the clarity back.