Installing a dimmer switch

I’m replacing a regular light switch with a new dimmer switch. When I pulled out the switch, there was a red and black wire that were slotted into the top of the switch, and then a thicker black wire slotted into the bottom. And the grounding wire. The new dimmer switch has a green and white wire – I know the green wire is the grounding wire but don’t know which of the black and red wire from the outlet is the neutral and which is hot. Also, on the side of the dimmer switch are spots for Load and Line. So I have 3 wires to connect to the white (neutral) wire, the Load and the Line on the dimmer…