Move player via button press with including acceleration / decelertion?

Dear Unity community,

I am a C# coder (beginner) but I use the 3rd person character controller from Unity which is java script for my project. So you may forgive me the stupid question:

Do you have some ideas how to edit the java script so that the player moves constantly when I press a button instead of just AS LONG as I push the button? There should be an acceleration / decelleration if I press the button more often (or the negative button), but I don't know the java script commands or how to start.

Is it possible to do this in C# too?

Thank you for your ideas and your help, guys! :)