Good posture in sitting

When you sit, the back of your knees must have space at least 3 finger from the front of the sit pan.

If your feet do not attaches the floor, or you are too short you must use the footrest.

Glasses

If you are wearing glasses, you need to tilt down your monitor 15-35 degree because, to reduces glare and lighting by minimizing the amount of light reflecting off the front and back surfaces of your eye glasses. 

Distance

The distance between you and your monitor should be between 60-90 cm. a good way to see if your monitor is far enough to sit in your neutral position (which the chair pulled in where you would normally sit) and straighten your arm in front  of you. If your hand touches the monitor, you are to close to the monitor.How far you sit from the monitor will depend on your vision, your age and the size and resolution of your monitor. It's the best to sit as far as possible and increase your font size. The distance from your monitor and you should be between 20-40 inches or 60-90cm. A good way to see if your monitor is far enough is to sit in your neutral position (with chair pulled in where you would normally sit) and straighten your arm in front of you. 

Sit a comfortable distance from the monitor where you can easily read all text with your head and torso in an upright posture and your back supported by your chair. Provide adequate desk space between the monitor (table depth) and the user.

Height and location

Working with your head and neck turn to the side for a long prolonged period loads neck  muscle unevenly and increases fatigue and pain. Monitors cannot be farther than 35 degree to the left or right (if your computer position is in front of you). If your monitor is too high you have to work with your head and neck tilted back. You can raise your chair until you can view the monitor without tilting your back. And if your monitor is too low, Monitor should be at or slightly below your eye level, normally it's located 15-20 degrees below your eye level. Downward viewing angle never greater than 60 degrees. 

The position of the monitor must be below your eye level and should be tilted 15 degrees for proper accommodation of your eye. 

Things to do

If you are sitting at a computer at a long periods, you can do these things to avoid injuries:

The maximum time you are sitting and working at your computer Is 2 hours. If you must work more than 2 hours, you can do some stretches shown above and rest your eyes by closing it for 5 minutes. You can go outside and take a walk.