how do you catch rotom and change forms???

To catch Rotom go to the Old Chateu between 8 P.M. And 4 A.M. And go upstairs to the room with the T.V. In it and thump it. Rotom will come out of the T.V. And will be level 15. If you knock him out it's Ok because he'll be back the next night. You can't change his Forme yet however, the Secret Key isn't available for downlaod until April 20.Not sure about that date though.

It needs to be between 8PM and 4AM at night and thump the tv in one of the room on the top floor.

You need the secret key you can go to Toys'r'us and get one or get it from Mystery Gift.

If you want to catch Rotom, you need to go to Old Chateu between 8 P.M. And 4 A.M. Go to the room with the TV and thump it. Rotom will imediately come out and battle. In order to change it's form, you need the secret key. Use the secret key to access a secret room in Team Galactic Eterna building. If you want the key, you have to first get access to mystery gift by answering the guy on the third floor of the Jubilife building's first question
EVERYONE HAPPY and his second question WI-FI CONNECTION. Then reset the DS system. If the title screen menu has a MYSTERY GIFT option, you have suceeded. In order to receive the key, you need to have WI-FI connection. Also, there are specific "event days" that you need to do it on. However, I am still unsure of when the "events" are. This is a very long process, I know. I hope this helps you!

Go to the forest befor the second gym. Look for a house. You must have HM cut and go into the house and look for a room whit a TV. Go into the room when it is 8 h PM. Talk to the TV (2x) clik yes and you fond rotom.