Google "[language/framework/technology] interview questions" and use that as a starting point. A lot of engineers tasked to interview candidates don't actually

Google "[language/framework/technology] interview questions" and use that as a starting point. A lot of engineers tasked to interview candidates don't actually know how to interview well on their end, and often will make the exact same google query to figure out what to ask. Also, be willing to talk about the work you have done in the past with confidence. Be able to say why it was interesting to you, and how you overcame challenges with it. If you feel week in the areas you mention (api design, class design, etc), then pick up a book and start reading away. The GoF design patterns also comes up in interviews a fair amount... not saying it should, but it does.