How do I know/calculate the number of coil turns I need?

[ENIAC]

I am trying to build a radio but first I need a coil. But I don't know what formula I need in order to calculate the number of terms need.

Please explain
Thanks

[w2aew]

The first thing you need to know is what value of inductance do you need?  This will depend on the frequency range being used, and the surrounding circuitry.  If you know the inductance, you can use one of many online calculators to figure out the number of turns.  Here's an example for an air-core inductor:
http://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml
There are others for toroidal inductors, etc...