How do i get charged creepers?

Solution 1:

According to the wiki

A charged Creeper is created when lightning strikes within 3 - 4 blocks of a normal Creeper

You will know it works when the creeper has a blue aura around it.

Getting the creeper struck by lightning is the hard part. You will likely need lots of luck if you want to avoid finding a mod that will charge one for you. You can see this answer for odds

If you want to use a mod, CommandBook can offer you Thor's hammer to call down lightning strikes with the thor <player> command. It is also Bukkit compatible.

Solution 2:

As David Starkley pointed out, a charged creeper occurs when lightning strikes near a creeper. Although possible, it is extraordinarily improbable that it happens naturally in front of your eyes.

However, you can increase those odds significantly using some tricks by YouTuber Docm77, who was able to pull it off without any mods, map editors, or even creative mode:

• YouTube: Minecraft - Producing Power Creepers (video showing off the results)
• YouTube: Docm77´s NEW Minecraft World Tour : Episode 76: Crazy Cats and Power Creepers (video where he explains a bit about the mob trap)
• YouTube: Docm77´s NEW Minecraft World Tour : 77th Episode Special (another video where he explains the mob trap. The explanation begins at around 1hr 5min.)

Solution 3:

To get charged (actually called supercharged) creepers, a lighting bolt has to land within 4 blocks of them. Also, via McEdit or other mods/clients, you are able to have supercharged spawn eggs and supercharged spawners.

Hope this helped!