Libraries for sequencing are prepared by adding adapters of a known sequence to the end of the DNA.

If you used pcr to get your product then the pcr primers will work well as sequencing primers. If the sequence is cloned into a vector then vector specific primers can be used to sequence the insert

You only need one primer for Sanger. As folks said, it can be from the vector you cloned into or the PCR reaction to create the product. And once you learn the sequence of part of your DNA of interest, you can use that to design more primers if needed to "walk" the entire sequence.

Talk with your advisor or a postdoc.

As stated above by Dr Rutland, PCR primers can also be used for Sanger Sequencing.