There are 2 ways. You can use sugar from your package manager or you can install the sugar build environment. If you install sugar build, you can change the shell code, where as if you install sugar normally you can only change activities.
Really, just find something that you are interested in in sugar! Once you have, look at https://bugs.sugarlabs.org to find bugs in it. You can also look for the code of the activity on github - there are links to githubs for all activities on the new activity library.
Feel free to chat more once you've found something interesting.
There are some docs which are wayy to complex, but they may help: https://help.sugarlabs.org/en/
The HIG also are super helpful in understanding why sugar is what it is. It really needs to be integrated into a welcome to sugar guide to make something really helpful, but yeah. Not every feature described is in sugar, and there have been some changes to make things better, but have a look: http://wiki.sugarlabs.org/go/HIG