Before anything else, speed-read (skim) through the whole passage, understanding what the passage is actually about. Through this, you would instantly get an idea of where the important information are and which bits to extract. This would allow you to properly uphold the main message of the whole passage through your summary.
Secondly, you return to the start of the passage again, already being well directed on where the vital information are from step one. This is when you “actively” read and underline or label the key bits and pieces, separating them according to which information should come first, or later, aren't necessary at all (repeated before). After this, you should have a proper draft of your summary ready.
In the cases of interpreting informative graphs and charts, focus on change. With attention and respect to variables (time, place, etc) calculate on what changes took place and always interpret it, instead of simply stating it down. For instance, a 5% increase in salaries from November to December. This method saves you precious words as well.
In most cases, accurately stating the change is not enough. You must also elaborate on what that change does and how it has an overall impact. This proves you have understood the context of the passage and have a complete view. For instance, a 5% increase from in salaries made profits have a downward (decreasing) trend between November and December.
Take it sentence by sentence, and don't be worried if you're taking this part slow. You're supposed to be, anyway. Learn to add transition words like “moreover,” “in addition,” “however,” etc, to keep a flow in your writing, or it may just seem like a salad of information, being as tasty as it sounds. It does have to read well, right?