You'll want to try to understand true airspeed before you worry about how to calculate it.
It can be said that true airspeed is the actual speed of the aircraft through the air. When the density of air decreases we will go faster with any given power setting. This is due to a decrease in air resistence (parasite drag) due to less dense air. With that being said an increase in altitude will increase your true airspeed do to the decrease in air density.
It is also true that hot days (high denisty altitudes) will also cause higher speeds, because the air density will also decrease.
But now your wondering why the airspeed indicator stays the same.
we can say that Indicated airspeed is only correct during ISA at sea level. as we increase altitude airspeed (TAS) will increase but even though we are flying faster through the air the decrease in air denisty results in less molecules entering the pitot tube. So even though we are flying faster, there are the same ammount of molecules entering the pitot tube due to the loss in air density.
Now once you understand that you simply calculate TAS using a flight computer with a simple altitude to temp conversion.
Hope I could help