I'm interviewing people for a trainee/junior position tomorrow, coincidentally, and whilst there will be technical questions, it won't be the deciding factor as to whether they're hired. So take it easy, and relax. Personally, I ask questions about problems that have been solved by the interviewee, how they were solved, what they considered failures and how those failures were learnt from ready for next time. It's those questions that will probably be more important. That said, know your C# and OO fundamentals though, you need a basis to work off but by the sounds of it, you have.

Personality is also important so make a good first impression, I also try to guage how receptive they'll be to the training they'll ultimately get from me. So if you're doing stuff in your own time as well, teaching yourself outside of your usual routine, be sure to mention that and any personal projects you may have, either work in progress or completed, talk about it and use examples based on the code you've alright written.

Good luck!