First of all, I would like to make it clear that Rotom will only appear on Tuesdays.
You have a chance of encountering Rotom by interacting with a shaking trash can.
If you find a shaking trash can, save beside it. When you interact with it, try and time your interactions with the shakes, as this helps in actually encountering a Pokemon.
Just keep interacting and soft resetting until you encounter a Rotom.
If you want movesets for Rotom, take a look at the moveset questions here.
Source: This video.
Hope I helped!
Edit: Another way of getting Rotom is to trade over the GTS, or arrange a trade with someone. This is the way I obtained my Rotom.