I'll answer both questions individually.
1) Moveset Format
A moveset format is usually done like this:
Pokémon@ Item
Ability
EVs/EVs/EVs
Nature
- Move 1
- Move 2
- Move 3
- Move 4
In your case, it would be
Castform@ Heat Rock
Ability: Forecast
(Insert EVs here)
(Insert Nature here)
- Sunny Day
- Fire Blast
- Weather Ball
- SolarBeam
2) Heat Rock Location
The Heat rock can be found at the Weather Institute on Route 119. It is a possible gift from the receptionist after defeating Team Magma/Aqua, chosen at random from the four weather rocks (I got a Smooth Rock).
Source (scroll aaaaaaaaaaallllll the way to the bottom)