there are several options. if you want to save the image to be put on the web, you can go to File>save for web> and in the bottom corner manually enter the size. and it will keep the proportions that you want.
another way is to go to image>image size
try these as well:
I guess the Image Size resampling method is set to "Bicubic Sharper" or "Bicubic Automatic" which will use "Bicubic Sharper" when downsizing (and "Bicubic Smoother" when upsizing). "Bicubic Sharper" tends to be heavy with the sharpening and create artefacts, such as halos, which many people find objectionable.
You may find best results by using standard "Bicubic" resampling along with sharpening which you control.
Some people pre-sharpen the full-size image, some post-sharpen the reduced image, and others do both. There are several sharpening methods available in menu Filter > Sharpen.
First try downsizing with standard "Bicubic" resampling to get a feel for how that looks without sharpening.
Which interpolation method is best for an image depends on the content of the image and how its been processed.
Some prior appen stated that Bicubic Sharper tends to be heavy handed and creates artifects. This does happen however it happens mostly because of how an image has been processed so far. It happens to images that have been highly sharpened.
Sharpeming is very persional and too much has been written about sharpening. Perhaps I take too simplistic approch. I sharpen Images at two points in post processing, First on input normally in ACR for RAW file a in NeatImage for other image file formats. The input sharpen I do is very mild. I just want a image a little sharper then I get straight from the camera. Gives me an image a little better to judge how well processing is going.
I Sharpen a second time for output. For Printing I add a sharpening adjustment layer that I can adjust the sharening effects and make different test prints.
For the web I have read the you should downsize then sharpen. Because my images are still quite soft I just use Bicubic Sharper and my images look great on my displays. How they look on users display depends on the quality of the user displays, its dpi resolution, how well its calabrated and the lighting in its environment. All of which is outside my control.
Nole suggest that you set your default interpolation to Bicubic. Bicubic interpolation works well on most images. If you look at the Adobe Plug-in script "Fit Image" you will see it always uses Bicubic up sizeing and down sizing. So Adobe also thinks Bicubic is a good general purpose interpolation method.