Your private tour guide will meet you in the lobby of your hotel holding a sign with your name on. Afterwards you will discuss what you will going to see. You can then set off for your tour and let your guide fill you in with all information possible.
It is highly recommended to inform the tour provider in advance about any special sightseeing wishes so that your admissions could be pre-ordered to ensure skip-the-line entry. Your tour operator will pre-order your tickets, so that you do not have to wait in the line (admissions however not included in the price).
As much as you can decide what you want to see, here are some highlights that you should not miss: Church on the Spilled Blood, St Isaac's cathedral, Peter and Paul Fortress, Hermitage, Yusupov palace, Russian Museum, Faberge Museum, Alexander-Nevsky Lavra, River and Canal tour etc... If you want, you can spend your entire day in Hermitage, or visiting Imperial residences outside of St Petersburg - like Catherine's palace and park, Pavlovsk palace and park, Peterhof palace and park or Gatchina castle and park. You will then be taken back to the point of departure.