Former President Richard M. Nixon has decided to do without the Secret Service detail that has accompanied him since he left the White House in 1974, and he plans to choose protection that will not by paid by the taxpayer.

Mr. Nixon has commissioned ''a complete study of his security needs,'' his assistant, John Taylor, said today. ''Once that's done, he will decide how to proceed.''

Mr. Nixon is the first former President to refuse Federal protection.

He notified the Government of his decision Friday in a note that was hand-delivered to Treasury Secretary James A. Baker 3d, whose department includes the Secret Service.

Mr. Taylor said Mr. Nixon's sole reason for the move was to save the Government money. The Secret Service will stay with him until private guards are hired, and at no time will the former President be unprotected.