I want to major in sociology for my second year but I am currently taking it in summer school, meaning I haven't got accepted into the program yet. What do I do in terms of registering for courses on Solar? Tonight is the course selection for my year. Does this mean I have to wait until Solar opens up after I finished my summer school course in order to pick my courses?

It's probably easier just to wait for tonight. Or you could just e-mail someone from the Sociology department, explaining your situation to them so that they may grant you a waiver.

do you mean like try to pick courses tonight? will it even let me without a major?

Right before you pick courses for the Fall 2012 term, you will be asked to pick your major. You may then pick your summer course (though I anticipate that would be a hassle since you would need to re-enter SOLAR multiple times)..