Andris Nelsons, new music director of the Boston Symphony Orchestra, will announce his first season at a press conference Wednesday at Symphony Hall — and give listeners a taste of what’s to come. Before making his announcement, Nelsons (above) will lead the BSO and 17 vocal soloists in a dress rehearsal of German composer Richard Strauss’s “Salome,” an opera perhaps best known for its final scene in which the soprano title character sings of her love for John the Baptist before kissing the lips of his severed head. The rehearsal precedes a one-night-only performance of “Salome” Thursday at Symphony Hall. Nelsons, 35, was appointed last May to be the BSO’s 15th music director since 1881. The Latvian-born conductor is BSO’s youngest music director in over 100 years. He succeeds James Levine, who was the orchestra’s music director from 2004 to 2011.