The most direct way to get expert-level help on math topics is to ask an expert - a professor, researcher, or strong student. Asking experts individually is not very efficient, so asking an expert-level question on a dedicated website, such as Math.StackExchange.com, usually yields quicker results.
The mathematical community at Math.StackExchange.com consists of researchers, professionals, professors, students and more. Stack exchanges are question and answer websites that are dedicated to specific topics. Questions on any expert-level mathematics topic are welcome at there, where professionals in all mathematical fields will be able to weigh in and share their answer. It can also be helpful to search the site for related questions that may have been addressed in the past.
Most mathematics questions are well suited for the website's community. Questions that are of specific interest to research-level mathematicians, such as new topics of study in a certain field or obstructions that a member has encountered in their research, may be better received at MathOverflow.net. Their community is more critical due to the specialized nature of the website, but still open to help with any question that meets their criteria.
For more in-depth help on undergraduate level math topics, one should visit KhanAcademy.org. The site offers hundreds of freely available instructional videos featuring detailed theory and instructional examples. If a question isn't outright answered, the relevant video should provide the tools needed to work through the solution.