Interview is just a process to analyse candidate's skills and expertise. Interviews are conducted to select right person for right job. If you want to prepare for interview, you need to improve your subject knowledge and skills required for that particular job. Now-a-days companies demand good communication skills so you need to improve your communication to get a job.
Interview Preparation can be done by yourself, no need to join any classes or institute and waste your money. You need to work on your skills and improve your communication. You can watch videos available on internet and can do self preparation. You can read books related to interview to overcome your fear for facing interview. The main thing required is you should be technically sound in your field and subject knowledge so that interviewer will be impressed by your knowledge and skills and you can get a good salary offer from the company.