How to make add to cart process smooth

Hi, I am working on shopping cart module but, I am facing some problems over there. Like when I want to proceed further after add to cart then, I have to sign in.
After sign in it goes to the dashboard and the cart items are vanished.

I wanna do that, after sign up user will directly redirected to the website and there items are also there at cart.

If there is any other way to do it please let me know.
I’m really stuck on this.

A Jolaniya