We don't do PTEs until late in the first week of class, but historically have been able to accommodate everyone who wants in. Make sure you come to class every day the first week.
If you are not in the School of Engineering, be aware that Program in Computing 10A is a class that covers some of what CS 31 does, although if you ask around, the word is that it is less intense than CS 31. If all you're looking for is an introduction to programming, PIC 10A is for you.