Refer to figure.
Learning Objective 062.01.02.01.01: Define ‘antenna’: an antenna or aerial is an electrical device which converts electric power into radio waves, and vice versa.

An electromagnetic wave is simply a constantly changing electric field and a constantly changing magnetic field travelling outwards from a source. Electric fields and magnetic fields are integrally linked, and happen perpendicular to each other due to Fleming's left hand rule.

Take a very simple dipole antenna for instance, if we apply a current in one direction, a load of electrons will begin to flow towards one side of the antenna, creating an electromagnetic field that wraps around the antenna. Then if we change the current to the other direction, all of a sudden the electromagnetic field will reverse on itself, and this ripple will travel outwards from the antenna, it is the beginning of a radio wave.

To create a radio wave, we just keep switching the direction of current in the antenna to make continuous back-forward-back-forward ripples which travel through space at the speed of light. Inside the antenna, the electrons are moving backwards and forwards at a frequency that we can decide, and which creates the frequency of radio waves produced. This movement of current is called alternating current (AC).

As for choosing the frequency, we know that we use VHF (Very High Frequency) radio waves for voice communications in civil aviation, which is between 30 and 300 MHz, so 30 000 000 to 300 000 000 "cycles" per second. That is much higher than the 50 Hz used by a lot of mains electricity supplies around the world.