You will need this:
A small box of baking soda
1/2 cup vinegar
At least 1 quart of hot water. Not boiling hot or you can crack the porcelain .)
I put baking soda and vinegar and use a plunger.i have two plungers- one for my sink and one for my toilet.
Here's a helpful video: https://www.youtube.com/watch?v=-7kE8JN7t4c