First of all, getting invited for an interview means that you are a good theoretical match for the job. The interview serves as a way to get to know you in person, to ask you questions that your resume and cover letter did not answer, and to drill down on what personality you are, i.e. whether you are really the best match for the job and the team. Therefore be prepared to answer questions about:
- your career history (it should all make sense; explain gaps and career changes; it should sound like you were working toward a goal, i.e. something like this job that you are applying for)
- you and your personality (expect the typical questions about your strengths and weaknesses, but be also prepared to give examples of work-related situations that show how you cope with problems, how you negotiate, how you solve problems, etc.)
- your future aspirations (be prepared to talk about where you see yourself in 2/ 5/ 10 years, what career path you want, how ambitious you are, etc. - but don't forget: if your aspirations do not match the career path or potential the vacancy has, you are not a good candidate anymore)
- your motivation (your potential new employer will ask you questions about why you left previous jobs, what motivated you to accept your current one, and of course, why you are the best candidate for the job you are applying for - obviously, your answer should never be "because this job pays better than my last" or "because I hate my current boss"!)
- your current job (there will also be questions about what exactly you are doing in your current job and whether this experience is relevant for the job you are applying for)
- your behavior (expect some pretty 'psychological' behavioral questions to gauge whether you would fit into your new work environment - these can range from "What was your biggest mistake and how did you handle it?" to "If you saw a colleague deliberately bending the rules, would you report them? Why or why not?" to "How would you handle a rude client?")
Finally, do not forget, that an interview is not a one-way street. You should also read up on the company that you want to work for and think of some questions you want to ask. Make sure you come across as very interested in the job and the company and ask intelligent questions about your tasks and responsibilities. Do not ask about remuneration during your first interview. If you appear to be a good candidate, they will invite you for a second, possibly even third, interview, and this is where you will learn more about remuneration, benefits, etc.
Overall, working with a trainer to prepare you for an interview is always a good idea. They will practice mock interviews with you, help you word the right answers, and make you feel more prepared.