2001 Chevy Camaro temperature light

for the past 2 weeks my temp gage has been moving into the red-at times almost touching the red. at firs it was on surface streets, but as soon as i got on the freeway it would level out. yesterday that didnt happen. i turned on the heat and it leveled off. before i go to a mechanic i want alittle bit of info so i dont get blind sided by info and cost.

Thank you