First you need to get the image up the editing screen.
Click "image" in the top bar and choose "resize" > "image size" from the drop-down menu.
In the table which now appears change "Resolution" to 72 (in the Document Size section of the table) and then change "width" and "height" (in the Pixel dimensions section). I usually set the width to 800 - that usually works okay.
Click "OK"
Then save the file as a JPEG.
When the "JPG Options" table appears slide the "Image Quality" bar along until the size (at bottom of table) is what you need. I usually aim for 150-200K
Click "Save"
Hope this all makes sense
Aaron