Decide what part of SXSW you want to attend. SXSW has grown from its original Music and Media Conference. It now also includes the SXSW Film Festival and the SXSW Interactive Festival, which focuses on media technologies.
Visit the SXSW website. You can find a description of the three different elements of SXSW on the homepage to help you decide which one you want to attend.
Click the "Register to Attend" link under the "Participate" heading.
Review the different levels of attendance described on this page. The Platinum Badge gives you access to all three events, while the Gold Badge gives you access to both the Film and Interactive Festivals. The Music, Film, and Interactive Badges each give you access to the single event. Finally, if you're a student, you can get a reduced-rate Student Badge that gives you access to the SXSW Film Festival.
Read the table to see how the price for your chosen badge increases over time. You can get a reduced rate for your badge up until about a month before SXSW begins. After that, you need to buy your badge at the walkup rate.
Determine whether the advance purchase rate is worth it to you. If you know you're going to attend SXSW and want to have full access to the events, purchase your badge in advance. However, SXSW doesn't provide refunds for badges for any reason, so only buy your badge once you know you can make it.