In setting up software, choose options giving text that is large enough to read easily on your screen, when you are sitting in a normal, comfortable working position. Individual characters on the screen should be sharply focused and should not flicker or move. If they do, the VDU may need servicing or adjustment. Don't sit in the same position for long periods. Make sure you change your posture as often as practicable.
Most jobs provide opportunities to take a break from the screen, e.g. to do filing or photocopying.
Make use of them. If there are no such natural breaks in your job, you should discuss arrangements to have rest breaks with your manager. Frequent short breaks are better than fewer long ones. The design of this equipment means that they are compact and generally have a smaller, fixed keyboard and screen. Portable computers should be used for short periods only with frequent regular breaks taken.