Sartre's existentialism revolves around a few central ideas.

• Since God doesn't exist, there are no pre-given objective values, and therefore no pre-given objectively good ends towards which divine providence shepherds the world.  Nor can abstract and allegedly universal ethical theories (e.g., Kant's), which could provide us with pre-set goals, deal with concrete lived cases such as that of the student who needs to chose between joining the resistance or supporting his mother.  In sum, we are in a state of "abandonment."  Hence, in the case of human beings, existence precedes essence. There's no human essence or model in the sense that we don't have an intrinsic function or goal (note the comparison with the knife).  I am what I make of myself in the sense that it's my choice that gives direction and purpose to my life; since values are not intrinsic fixtures of the world, I must invent them. Such choice/invention need not be, and usually isn't, the result of a conscious deliberative process, but it is always an indication of man's dignity and radical freedom. This self-legislating necessity (the result of abandonment) is the humanism of existentialism.
• Existentialism emphasizes the role of the subject in determining one's essence.  However, this radical form of subjectivism is tempered by two considerations:
1. "Every truth and every action imply both an environment and human subjectivity," in the sense that what we believe, choose, and do is the result of the attrition between two forces: our subjectivity, self-determining powers, and freedom on one side, and the environment, be it natural or social, on the other.  Still, all in all, the primacy seems to belong to the subject, who can invest his 'facticity' (what he has been up to now, his historical situation, one might say) with a new meaning, and therefore transform it through his 'transcendence' (self-determining capacity).  So, the would be Jesuit, in Sartre's own example, interprets his failures in life as a sign of religious calling.
2. When choosing, I choose for all humankind in that I implicitly set myself up as an example which others ought to follow: "in fashioning myself, I fashion man." This is a profound responsibility which generates anguish and diminishes the probability of capricious choice. Furthermore, since there's no divine providence I can work along with, it makes no sense for me to rely on that which is totally beyond my control.  Abandonment, then, generates despair, which is, nevertheless, more than balanced by the optimistic consideration that, in making ourselves what we are, we are our own masters .
• Since one's essence is given by one's self-given purpose and goals, one's existence, one's being, is just the sum of the actualization of such purpose and goals, that is, one's actions and accomplishments. One is a coward or a hero because of what one does, not because of how one feels, or one's temperament, or one's circumstances. Denying this amounts to misunderstanding one's radical freedom and falling into self- deception (bad faith).