Question
What type of soil do they need? How can I provide
that type?

Thank you

Answer
Gwen,
Gardenias prefer well drained, acidic soils that are high in organic matter.
Use a high quality potting mix and mix in an equal amount of compost. To maintain the acid level, use a product such as Muracid.
I hope this helps.
Good luck.
Ed Gulliksen