How can I learned interview skills?

My work record is a little spotty. I have worked two jobs that they had me there for 20 hrs with only 5 hours of work. My best reference is as a janitor for a school I worked at and I have a bad reference at a constuction company where I worked as a receptionist and started having problems when the workers started giving me a hard time. I want to get back into an office as a CAD Drafter. I have been taking cles at night and have had a couple of interviews but get a little tougue tied when it comes to my work experience.