just use the Assistant to create a (standard) VPN. "Tunnel mode" is the default mode of an IPsec VPN (as opposed to "transport mode" which is e.g. used in conjuction with L2TP).
If you have more questions, please have a look at either the Admin Handbook or the Cookbook. The CB lists solutions for common scenarios without too much explanation. You can download both PDF documents at docs.fortinet.com .