Probably the two main options would be:
1. Get a book that covers some more advanced topics or topics you've covered but in greater detail. Books are less popular these days but still a good option if you can absorb them.
2. Come up with an idea for a project and try to implement it, then look for specific information, either existing or in answer to questions of forums like this, when you encounter specific issues along the way. C# programming is such a broad topic that it's hard to know what you need to know until you need to know it.