How to change nameservers?

Hi! I have a domain name registered with Iwantmyname (rustless.co) and I want to point it to my Altervista account. Where do I go to do this?

Also I heard that all Altervista does is a kind of redirect, so you enter rustless.co and you end up at rustless.altervista.org? Is there any way around this?

Currently this is not possible, you can not bind (nor transfer) an external domain to your AlterVista website.
If your current DNS hosting service allows to setup an HTTP redirect for your domain you could use that.