To contribute, simply make a pull request.
Make a folder for the Programming Language you are using if there isn't already one. Make sure your code is correct too. Then, make a file that is problem-name.file_extension. So, for example, if I had a solution for 2048 in Python, then I would make 2048.py. (The name is the text that goes after "https://dmoj.ca/problem/" so if my url was https://dmoj.ca/problem/my-very-cool-problem, then I would write a C++ solution named my-very-cool-problem.cpp. Make sure to put your file in the correct directory (folder)!!!
When contributing, you can either add a new problem or complete an already existing one (denoted by a comment saying not yet completed). There are also some issues you can solve.