Siman 125
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א אֵין הַצִּבּוּר אוֹמְרִים עִם שַׁ"ץ נַקְדִּישָׁךְ, אֶלָּא שׁוֹתְקִין וּמְכַוְּנִין לְמַה שֶּׁשְּׁלִיחַ צִבּוּר אוֹמֵר עַד שֶׁמַּגִּיעַ לִקְדֻשָּׁה, וְאָז עוֹנִים הַצִּבּוּר קָדוֹשׁ.
1 The Laws of Kedusha. Containing 2 S'ifim: The congregation does not say "Nakdishakh..." [the opening of the Kedusha] together with the prayer leader, rather they should be silent and concentrate on what the prayer leader is saying, until [the leader] reaches Kedusha [proper], and then the congregation answers "Kadosh".
ב טוֹב לְכַוֵּן רַגְלָיו בְּשָׁעָה שֶׁאוֹמֵר קְדֻשָּׁה עִם שְׁלִיחַ צִבּוּר. הַגָּה: וְיֵשׁ לִשָׂא הָעֵינַיִם לַמָּרוֹם בְּשָׁעָה שֶׁאוֹמְרִים קְדֻשָּׁה, וְכֵן מְנַעְנְעִים גּוּפָן וְנוֹשְׂאִין אוֹתוֹ מִן הָאָרֶץ (טוּר וְשִׁבּוֹלֵי לֶקֶט); וְאֵין לְדַבֵּר בְּאֶמְצַע הַקְּדֻשָּׁה (מַהֲרִי"ל וּבֵית יוֹסֵף בְּשֵׁם מַהֲרִי"א וְדֶרֶךְ אֶרֶץ זוּטָא). וּמִי שֶׁאָמַר סֵדֶר קְדֻשָּׁה, וּבָא לְבֵית הַכְּנֶסֶת וּמָצָא צִבּוּר עוֹנִין קְדֻשָּׁה, חוֹזֵר וְעוֹנֶה עִמָּהֶם (תְּשׁוּבַת הָרַשְׁבָּ"א סי' רמ"ט).
2 It is proper is pay attention to one's feet [i.e. keep them together] at the time when one is saying Kedusha with the prayer leader. Rema: And people should raise the eyes on high at the time at which they are saying Kedusha, and also move their body and lift themselves from the ground [Tur and Shibolei Leket]. And one should not speak in the middle of Kedusha [Mahari"l and Beit Yosef in the name of Mahari"a and Derech Eretz Zuta]. And one who [already] said the order of the Kedusha and came to the synagogue and found the congregation answering Kedusha should repeat it and answer with them [Responsum of the Rashb"a - siman 249]