“The Oxford 3000” (PDF listing) is a selection of the 3000 most frequently used words in English. If you know them, you can understand the meaning of more than 80% of the words in a general text.
Is there something like that for French? A simplified dictionary for students would be nice too. Because I'm in a condition in which I cannot buy any book, I would prefer to find such dictionary online or in PDF format so that I can print it.