This site is not recommended to you for the software: Although TV Paint is a great program!
It is recommended because of the excellent insight into the process of animation you will find there.

http://tvpaintanimation.tumblr.com/post/12549832049/this-is-really-cool-i-love-this-one-thing-about

Read Chelea’s post on how to determine Breakdowns and Inbetweens based on established Timing and Extremes and if at the end of it all this timing chart business still isn’t making sense, that is okay.
Just let it sink in. It will someday.

Apologies to Chelsea if the editing is too heavy handed in the PDF!

TIMING CHARTS – Determining Inbetweens by the Timing and Keyframes