I agree with Alicia completely. Read a lot. But in addition, I would add that it would help to practice trying to break down whatever you read into its main components (in particular, break down the argument being made, or the main idea being expressed and its support). SAT reading sections will be about comprehension of what you've read, and this often (i) comes in the form of questions about the main idea/topic/these/etc. or the supporting claims, and/or (ii) will rely on understanding the underlying argument, thesis, topic, support, etc.
A good way to practice learning how to break down what you read in this way--such that you understand what is being argued or expressed, how it is, what support is being given, what the main point is, and so on--is to do some introductory logic work. Find an intro to logic textbook and study the sections on the fundamentals of arguments, recognizing and analyzing arguments and their components, constructing arguments from passages, and so forth. These texts typically have sections dedicated to these, and they will almost always have practice questions at the end of each section, including being able to practice breaking down small passages into conclusions and supporting premises, finding fallacies, etc.
This will also help with the writing section on the SAT, too. Good luck!