RA positions provide tuition credit and a stipend and the opportunity to work with a faculty member and their other RAs on research projects.
There are a very limited number of RA opportunities each year, and the positions are highly sought after. Occasionally they are announced on the Design Group's job posting email distribution list and website, but in general there is no list of available positions. They are granted by individual faculty members. If you are interested in an RA position, please contact the faculty member you would like to work with.
It's important to note that students are rarely offered positions in their first quarter at Stanford. Faculty members usually offer RA positions to students they have worked with in class or have talked with in some detail.
If you are interested in obtaining an RA position, here is a plan of action you might consider:
Take as many seminars as you can to learn about the various faculty in the Group and Department and their research projects.
When you've identified faculty you might be interested in working with, ask them if you might sit in on a meeting of their research group. This will be a great way for you to get first-hand knowledge of what goes on in these meetings, and to see if you think being an RA is for you.
Narrow the field to the faculty member you most want to work with and ask if you can do an independent study project with the faculty member as your advisor. A quarter spent in this way will give the two of you a chance to get to know each other both professionally and personally.
Perhaps your efforts will lead to an opportunity for an RA position in this or a future quarter.