The Gemara (Yoma 81b) learns from the laws of Yom Kippur that there is a Mitzvah to accept Shabbat upon oneself earlier than the official time and thereby “adding” to Shabbat as it were. There is a disagreement if the Mitzvah of Tosefet Shabbat (lit. “adding to Shabbat”) is biblical or Rabbinic, but in any case, the Shulhan Aruch (O.H. 261:2) writes that it is a worthy act to take on Shabbat early. Although there is no prescribed amount of time that one should begin Shabbat early, it is commendable to cease forbidden labors and accept Shabbat approximately three or more minutes before sunset.
The Mishna Berura (M.B., O.H. 261:21) cites an opinion that maintains that one should make a verbal declaration that one is accepting Shabbat. Rabbi Shalom Messas (Shemesh Umagen, vol. IV, § 41) on the other hand, remarks that one need not declare verbally that one is accepting Shabbat, but rather the mere fact that one ceases forbidden labors is sufficient in fulfilling Tosefet Shabbat. This is also the opinion cited in the book Meged Giv’ot Olam in the name of Rabbi Moshe Feinstein.
Although Rabbi Ben Zion Abba Shaul (Or LeZion, ch. 18, § 2), based on the Ritva (Shabbat 23b), writes that ideally one should verbally declare that one is accepting Shabbat, nevertheless it is possible that the Ritva is referring to a case in which one is not reciting Kabbalat Shabbat and thus would need a firm reminder and declaration of accepting Shabbat. Nevertheless, for one who is present at synagogue, the recital of Shir Hashirim and the rest of Kabbalat Shabbat is in and of itself a declaration of accepting Shabbat.
Summary: It is a Mitzvah to accept Shabbat early. One need not accept Shabbat verbally.