You are in for the experience of a lifetime visiting Sevilla for Semana Santa.
I visited during this period first in 2003 thinking it was my "once in a lifetime" trip but I'll be returning next month for the 4th year in a row!
I found a website that may be helpful to you. It's only in spanish but my spanish is limited and I can muddle through it :o .
web page I usually try to get a guide at the Santa Justa train station when I come in on the AVE from Madrid. They also have guides in the tourist office and some newspaper kiosks but sometimes run out later in the week. The daily paper, ABC, also lists the processional routes for the evening along with the times.
Hope this helps and have a wonderful holiday.