Is there any one step checkout or fast checkout addon?

There are too many fields to fill up when customers checkout.
is there any addon to simplify or minimize the process?

There is Fast and Easy Checkout in Plugins, and also COWOA to make customer accounts optional. Neither of them substantially reduces the actual amount of information customers need to enter; the process doesn't ask for information that is not (normally) needed. FEC and stores like Amazon don't actually reduce the amount of info entered, they just collapse and package it in a way that makes it convenient (and Amazon uses cookies to track and remember who you are and what you looked at last week).

The question is what is wrong with the zen cart checkout actually. Please take time to read my blog post about this before you go installing checkout mods. http://www.teamwiztech.com/blog/whats-wrong-with-zen-cart-checkout/

A checkout is a checkout - you got to get a certain variety of information. Someone tried to do a true one page checkout years ago and that one page had everything one it - a very confusing and inefficient checkout. Oh, and it never worked!