When reading scientific material, it is easier to learn if you skim for basic concepts first. This allows you to have basic theories in mind before getting bogged down in details.
Read in topic sections and review each section before moving on to the next sub-heading/chapter/paragraph. This is helpful because most science texts build from general to specific as the chapter goes on. When done reading, ask yourself "What did I get out of this?"
When finished with the reading assignment, skim through again and take brief notes on important concepts. This makes a great study tool for reviewing the readings without having to read through the chapter again and determine what is important the day before a test.
After completing the reading, write down questions on any of the topics and ideas that you found challenging to ask questions on later.
If you get bogged down with details, check the chapter summary to see if there is a simplified version of the concept so that you can get an idea of what it means before trying to understand all the details.
Only take notes on major concepts: rewriting the chapter is an ineffective use of time and makes quick reviews very difficult.
Read and Review topic breaks
Read/Skim and take topic notes on what you believe are the important points.
Make a list of especially challenging questions and topics/issues to ask questions about or re-read later.