I only got involved with open source (and sympy) in February and I found that looking at bugs and trying to solve them is the best way to get started. Since you applied for GSoC this year, you probably submitted a fix for some bug as part of your application? Well, try another one. Find something that looks interesting, try to work out why the bug happens and ask for help if you are unsure how to fix it once you found the problem. You'll learn things while trying. And if you can't solve it, maybe someone else does and you can see what they did and learn from that too. You have probably already read this (and should do if you haven't):
https://github.com/sympy/sympy/wiki/Introduction-to-contributing . And the "Easy to Fix" bugs are here:
https://github.com/sympy/sympy/labels/Easy%20to%20Fix but feel free to try something without this label if it looks interesting.