Primary residence turned into rental advice

[Phil Truong]

Hi Everyone, 

looking for some clarity on captial gains on a primary residence that I will  be renting out after I move into my new house.

if I decide to sell my first house that I turned into a rental after 5 years, will I pay based on The difference of fair market value at the time I turned it into a rental or all of it?

Also should I get it the primary turned rental appraised?

any advice is much appreciated !