The conventional day tour leaves Huaraz [1] at 9:30 a.m., returns at 7:30 or 8 p.m., costs between US$10 and US$12 pp, and includes transport and guide. Bring plenty of warm clothes, bag lunch, and sun protection, and make sure your guide speaks English well.
The most reliable tour operator is Pablo Tours (Luzuriaga 501, tel. 043/42-1145, www.pablotours.com [2]).
Other options are Chavín Tours (Luzuriaga 502, tel. 043/42-1578, www.chavintours.com.pe [3]) and Sechín Tours (Julián de Morales 602, tel. 043/42-1419, www.sechintours.com [4]).
Although these agencies offer other trips, their services are only recommended for conventional tours.
Links:
[1] http://www.moon.com/destinations/peru/huaraz-and-the-cordillera-blanca/huaraz
[2] http://www.pablotours.com
[3] http://www.chavintours.com.pe
[4] http://www.sechintours.com