Do all what was said:
1.- On google webmaster tools register both sites, the new one and the old one. Then there on the configuration menu, on the old site specify that the site will be changed to the new one.
2.- Redirect all the URLS that pointed to the old site, to the new site, with 301 redirection. Depending on how your new site will be, you can either redirect each old URL to the new URL just specifying 301 URL REDIRECT on the host records to the new site, or redirect all the old URLS to the homepage of your new site, again on host records making a 301 Redirect URL, but without slash (this symbol "/") at the end of your new address, so everything will be redirected to the root of the new domain.
301 redirections also redirects human visitors, so do not worry about that.
I make a transfer like this one, and to recover the slight consequences, I made some SEO campaigns. I recovered after 1 month and after 2 months I was actually better (a lot much better) than before. So do not worry too much.
