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
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).