Por and Para can cause a lot of problems for the English speaker, especially when we pigeon hole both words as meaning ´for´. In reality the uses of each can be far more nuanced, specifically ‘Por’.
I would advise you to focus specifically on the uses of ‘Para’ though as these are fewer and in my opinion are slightly more logical, you can then almost assume that most other uses are served by ‘Por’ (it is also advisable to learn some of the set expressions and idioms used with ‘por’, they are many)
Have a look at the following links:
Some practice exercises: