How to connect my vlan to my forigate 80c ?

on the port that connects the FortiGate to the Switch you need to add VLANs. So for instance, you can click internal1 and then click create new (visible on your screenshot) and vlan is an option. Define VLAN parameters as needed.

From there that gives the interface the ability to see the traffic.

Then, whatever port connects the switch to the Gate (on the switch side) needs to be configured to trunk that vlan up.

Another option would be to have certain ports on the switch be part of vlan x (set this vlan as those ports default vlan not a tagged vlan) and then assign a physical interface on the gate to that selection of switch ports (plug internal2 on the Gate into port on the switch that is a part of the ports that have a new default vlan) Then it can hear that accordingly (you will need to setup the interface on the Gate's IP address for this network) Probably in your interest to make the FortiGate the default gateway for the vlan so you can do cross vlan inspection as well.