What was Nikola Tesla first idea?

Answer 1

Nikola Tesla states

there is a possibility of obtaining energy not only in the form of light, but motive power, and energy of any other form, in some more direct way from the medium. The time will be when this will be accomplished, and the time has come when one may utter such words before an enlightened audience without being considered a visionary. We are whirling through endless space with an inconceivable speed, all around us everything is spinning, everything is moving, everywhere is energy. There must be some way of availing ourselves of this energy more directly. Then, with the light obtained from the medium, with the power derived from it, with every form of energy obtained without effort, from the store forever inexhaustible, humanity will advance with giant strides. The mere contemplation of these magnificent possibilities expands our minds, strengthens our hopes and fills our hearts with supreme delight. . . . - "Experiments With Alternate Currents of Very High Frequency and their Application to Methods of Artificial Illumination," 1891

that was his primary idea since he was a boy. If you are talking about his first invention, it was the tesla telephone repeater.

Answer 2

One of Nikola Tesla's first ideas was to develop an alternating current (AC) electrical system, which would eventually revolutionize the way electricity is distributed and transmitted. Tesla believed that AC was more efficient and practical than the direct current (DC) system advocated by Thomas Edison.