We prepared a good course to prepare to study in MSc programme. Listen it in summer before the classes begin. https://www.coursera.org/learn/mathematics-for-economists
If you feel you need some more knowledge in micro, use https://www.coursera.org/learn/microeconomicspart1 and https://www.coursera.org/learn/microeconomics-part2. Listen it during the summer period or intensively in September. When the course of Advanced Microeconomics by Prof. Fridman starts, you will realize why I strongly recommend you to take a prep course of micro.
If you have no background in probability theory and econometrics, I recommend you https://ru.coursera.org/learn/introductiontoprobability or, if you speak Russian, https://openedu.ru/course/hse/METRIX/.
If you still know nothing about finance, take https://www.coursera.org/learn/wharton-finance or https://www.coursera.org/learn/corporate-finance-essentials. This course will help you to feel comfortable in class of Advanced Corporate Finance by Prof. Ivashkovskaya.
If you are not so good at coding, take a course of Python, e.g. https://www.coursera.org/learn/python-statistics-financial-analysis
Besides, I wish you to be really open-minded, so take courses on different things you don’t know. If you are going to live in the investment and finance world, first of all, I recommend you to study neurosciences (e.g.https://www.coursera.org/learn/neuroeconomics and maybe also https://www.coursera.org/learn/model-thinking).
In real world we almost every day work within a team, so you should develop your teamwork skills (https://www.coursera.org/learn/leading-teams?specialization=leading-teams could help). A basic course of psychology helps us to understand people and their motives better (https://www.coursera.org/learn/introduction-psychology).