Heating and cooling

Hey so encounted first ever problem as from daily driver I parked and took off for 2 months truck still parked same place and cranked up up good.
On my first drive checked fluids as I always do all good and suddenly warning appears for coolant over heating and as the warning goes of temperature drops quick and in 1hour drive it happened twice when checked coolant level dropped by a litter and reserve was full sorry no pictures. on my way back everything was good tho I drove crazy on stick shift everything was fine so I parked it again for a week when returned to see what’s going on
Just let the engine run on idle and reserve coolant increased
1. Before I let anyone touch any bolt is it a thermostat?
2. If it is shall I change water pump too or only thermostat
3. Most important I missed my drive belt is making noise could that be the problem it’s not letting it circulate right?
Any guid will help a lot please
Thank you all