As long as you can't block the major light sources easily, then you need a Monitor filter (wikipedia), which has anti glare or anti reflex/reflective behaviour. These can be bought commercially for at somewhat steep prices, or different DIY solutions available at cheaper prices.
Do a google search for anti reflex screen to get you started, which reveals both some commercial options and a DIY anti glare solution.
Suggestions for the DIY could be to use something called quilters plastic template, which can be overlayed onto your screen and make it quite a lot better. If you have some translucent thin plastic, it is well worth to try it in front of your screen.