The Shulhan Aruch (Orah Haim 22) rules that one who purchases a four-cornered garment and affixes Sisit to the garment recites the Beracha of Shehehiyanu. Generally, the Shulhan Aruch was of the opinion that the Beracha of Shehehiyanu recited over the acquisition of new items should be made at the time of the purchase. In the case of a Tallit, however, if a person purchases a Tallit without the Sisit, the garment is incomplete and unusable until he affixes Sisit. In fact, it is forbidden for him to wear the garment before he ties the Sisit to its corners. The Tallit at the time of the purchase resembles material that one purchases for the purpose of making clothes. Quite obviously, he experiences joy not at the time of the purchase, but rather at the time when the garment is completed. Therefore, in the case of a Tallit, too, the Shulhan Aruch rules that one recites Shehehiyanu after he affixes the Sisit, at which point the garment is suitable for wearing.
Regardless, our custom is to recite Shehehiyanu over a new garment when wearing it for the first time, and not at the time of the purchase. This applies to a Tallit, as well; one recites Shehehiyanu over a new Tallit when he puts it on for the first time, and not when he purchases it. He recites the Beracha of “Lehit’atef Be’sisit” followed immediately by Shehehiyanu. However, if one recited Shehehiyanu at the time of the purchase, he has certainly fulfilled his obligation and does not recite the Beracha again when he wears it for the first time.
The Mishna Berura (commentary by Rav Yisrael Meir Kagan, 1839-1933), commenting on the Shulhan Aruch’s ruling, notes that the Beracha of Shehehiyanu is recited over the actual Tallit, and not on the Misva of Sisit. We do not recite Shehehiyanu over the Misva of Sisit because it applies every day. We recite the Beracha of Shehehiyanu over events that generate excitement, such as a significant purchase, and when performing infrequent Misvot, such as Shofar on Rosh Hashanah and Lulab on Sukkot. The Misva of Sisit, however, is one which we observe every day, and it therefore does not warrant the Beracha of Shehehiyanu. Thus, the Mishna Berura writes, when the Shulhan Aruch requires reciting Shehehiyanu over a new Tallit, the Beracha relates to the garment, and not to the Misva.
This is in contrast to the ruling of the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), who held that one recites Shehehiyanu over a Misva that he performs the first time in his life, even though he will then perform the Misva each and every day. According to the Rambam, a Bar Misva boy should recite Shehehiyanu the first time he wears Tefillin, because he fulfills this Misva for the first time. Halacha does not follow the Rambam’s view in this regard, and thus one does not recite Shehehiyanu when laying Tefillin for the first time. However, it is advisable to instruct a Bar Misva to have the Tefillin in mind when he recites Shehehiyanu over his Tallit on the day he becomes a Bar Misva, in order to satisfy all opinions. (Of course, if he did not have his Tefillin in mind when he recited Shehehiyanu over his Tallit, he does not recite the Beracha over his Tefillin, because, as mentioned, Halacha does not follow the Rambam’s ruling on this issue.)
The Mishna Berura further notes that the Shulhan Aruch’s ruling applies to the Tallit, and not to the Sisit strings. If a person has been wearing a Tallit for several years, and now decides to replace the Sisit strings with a new set of strings, he does not recite Shehehiyanu the first time he wears the Tallit with the new strings. Shehehiyanu is recited only over a new Tallit, and not over new Sisit strings.
Summary: One who purchases a new Tallit recites Shehehiyanu when he wears it for the first time, immediately after reciting the Beracha of “Lehit’atef Be’sisit.” One who replaces the Sisit strings on his Tallit does not recite Shehehiyanu the first time he wears the Tallit with the new strings.