If you go into the "Video Settings" or "Graphic Settings" in your video game and it will not let you change the settings to high, it is because you either have your hardware acceleration set to low or your graphics card isn't good enough for the game. So you might need to get a new graphics card.

Try increasing your hard ware acceleration first (in Windows) by right clicking on your desktop, click "Properties", then click the "Settings" tab, then the "Advanced" button at the bottom. Once the "Advanced" box opens up, click the "Troubleshoot" tab.

There you will see a hardware acceleration slider, and it should be all the way at "Full". If it is, or you have already done it, and it still doesn't work then I would say you need a new video card.

You can learn more about buying and installing a new video card by clicking here.