How do I turn on powered rails?

Powered rails need to be powered to work. Use a redstone signal to power them. you can use a redstone block next to, or beneath the powered rail, or use a redstone torch next to the rail.

Another method is to put a detector rail in front of a powerd rail. This will send a redstone signal into the powerd rail.

A lot of this stuff can be found on the Minecraft wiki

You can activate powered rails by either powering them with redstone, or using detector rails.

Redstone torches are the easiest way to go, and will power the rail if they are adjacent to the powered rail, or underneath the block the rail is sitting on.

Detector rails can be placed in front of powered rails to power them, but only when a minecart rides over the detector rail.