Rotom Elevator:
Note* Obviously you must have a Rotom. You must have beaten Red, and Obatained Kanto and Hoen Starters

You can obtain Rotom from trading, Migrating.

How To get One From Pearl:
Note* You need Cut, And National Dex
Goto the Old Chateau which is located near the exit of Eterna Forest, use cut on the tree, walk into the Mansion.
Look around until you find a Small room with a TV, walk up to the TV and press A, you will automatically get into a fight with Rotom.

Rotom Elevator:
Note* Have Rotom first in your party.
Goto Silph Co. which is locate in Saffron City, Kanto Region.
Walk into Silph Co, Walk to the left, Go up to the Elevator
(If the Man is Standing there, It means you havent completed the Requirements, and or talked to Steven (The Guy to the very Right of the room standing at the counter))
If not, walk in.
Inside there will be five everyday household appliances click on one for your Rotom to transform into one.
Your Rotom will learn a Move every time it transforms, but will lose the move from the previous transformation