IDE:
Enhance your development environment as much as possible so it can give you feedback you can use.
Read, draft the struct on the paper of ppt
Practice:
Practice, practice, practice. The more code you read, the faster you will be at it
Also very important is that you have a great programming logical abitlity which comes only with practice. The more you practice coding and its execution, more will your logical ability improve. And the more you try to write each code in several different ways, more will you learn to think differently for a solution. With all this practice no one can stop you from reading a code well.
Refer good document
If there are good documents that provide overviews of the system, start there. Only if they are good, though.
Witting test code
When you really need to understand the code, and it is complex, writing some tests is invaluable. It helps you understand the possibly complex behavior, as well has being useful moving forward when you need to modify it.
Read Working Effectively with Legacy Code