Questions

That depends on the program you are using.

In Photoshop:
1 - open the Image, then go to Image > Image Size -make sure you have selected Pixels (not Percent) and the Height / Width ratio is locked (the chain icon is shown) (Constrain Proportion and Resample Image boxes are ticked).
2 - type the number of pixels required in the Width box or the Height box (generally you do not want the Image to be wider or taller than 700-800 pixels)
3 - click OK
4 - save the resize Image under the appropriate name and format (for most photographic images .jpg format is used - quality High or Medium (7-8), for graphic, flat colour images .png or .gif is better.

Note that:
The Resolution of the image (pixels/inch, pixels/cm) does not matter, only the pixel Dimensions.
To show on the Internet the image must be in RGB colour space (8bit or less) - not CMYK.
To be saved as .jpg the image must not contain Layers or Alpha channels.
Do not use spaces or reserved characters ( * , / \ | ? " ' @ # ~ ` ) in the name.

These Notes also apply to other programs although the steps to follow might somewhat vary.