What is the History of the title 'Naidu'?

 Historically, It has more than 1000 years of history as people who were soldiers or chiefs of the soldiers were called Nayakas during the ancient times and then those Nayaka got corrupted form as Nayak which eventually become Nayakudu and then 'Naidu'. 

Which caste started using it first?

Historically there is more than 1000 years of history to prove that it was Kammas who used it for long, but lately several other castes too started owning it to their name, below are some of the links provided to prove the same...!!! 

1. https://en.wikipedia.org/wiki/Nayaka_dynasties
2. https://en.wikipedia.org/wiki/List_of_Nayakars
3. https://en.wikipedia.org/wiki/Madurai_Nayak_dynasty
4. https://en.wikipedia.org/wiki/Pemmasani_Nayaks

Prominent names from the medieval history

1. Vasireddy venkatadri Naidu - 17th Century
2. Pemmasani Ramalinka Naidu - 15th Century
3. Gurijala Nayakas - 14th Century
4. Musunuri_Nayakas - 13th Century
5. Ravella Nayaks 13th Century
6. Pinnama Nayudu - 11th Century - Kakatiya Dynasty