There should be an Auto-Rotation switch in your quick access panel (in your expanded notification bar). If you don't have a quick access panel, you'll need to find the Rotation option (usually under Display) in your device's settings. There you can turn it on/off for auto-rotation or force either landscape or portrait mode.
HOWEVER not all apps support rotation (so even if you have it on Auto, if the app doesn't support that mode, the display will not rotate). Furthermore, some apps don't support inverted rotation (will only rotate with the bottom of the tablet always being 'down').
There are apps in the Play Store that can try to force screen rotation on apps that don't natively support it, but you might encounter glitches if you use apps like that (for instance, Instagram glitches out if you force it to rotate).