Best way to get from Sorrento to Amalfi

Carol, "best" is up to individual preference.

You basically have three choices:
SITA bus (most economical):
http://www.positano.com/en/bus-schedule

Jetboat:
http://www.positano.com/en/ferry-schedule?from=sorrento&to=amalfi
Note that not many jetboats make this journey but it's often the preferred method.

A third method - private transfer - is the most expensive but some folks do it, especially if they have a lot of luggage and/or have a big enough group to make the price more palatable.