Buy a few books for the subjects you have shortlisted. But take them-at the medium level, not advanced. These books should be of graduation level (first or second year) and they should cover the basics and core knowledge about the subject. Also, these books should not be too analytical in nature. They should be written by ordinary authors in a simple manner.
Now read the book of a subject. Take at least three four days to read it thoroughly. You can jot down important points, such as 'whether you liked studying the subject or were you bored? Did you find the subject easy or difficult? Was it easy to understand?' Now try recalling the topics that you read and see,how much of it do you remember. Write all this down somewhere.