Firstly, SAS programming skills(Data manipulation、Statistical procedures、Reporting and Graphing).
Secondly, Knowledgement to standard of this industry, that is CDISC.
There are multiple books of SAS in clinical trials, you can try: <Modern Approaches to Clinical Trials Using SAS>, <Implementing CDISC Using SAS>, <Clinical Graphs Using SAS>.
Thirdly and most importantly, get yourself an internship, you cann't become a real clinical data analyst without doing its work.