Paul, Silvanus, and Timothy, To the church of the Thessalonians in God the Father and the Lord Jesus Christ: Grace to you and peace. (1 Thessalonians 1:1 - ESV)
Paul and Silas (Silvanus) had left Philippi and entered Thessalonika. After being there and ministering, he had been basically forced to leave the city. Paul is now in Corinth and thinking of the church that was planted in Thessalonika, missing them, worrying about them, praying for them, and writing a letter to them.
He begins his letter to that church in the way he most often does. He includes who he is with, now joined by Timothy, and greets them in the name of the Father and the Son, hoping that God's Grace and Peace are on them.