Aristotle said "The function of man is activity of soul in accordance with reason...(Ethics 1.7). This quote can be a starting point to answer the very complicated question "How should we live?".
Aristotle believed that things are done to achieve some end; I study to get good grades, I work to earn money so I can get what I need etc. For Aristotle, even though our actions are done for the sake of another, eventually this "chain" has to end. It cannot go on forever or there would be no reason to act at all. Aristotle thought that there had to be something that we want "for its own sake".
What is this thing "for its own sake" that we want? For Aristotle, it has to be something that cannot be taken away, it has to fit the person. Since the most important thing that humans have is the ability to reason, whatever we seek for its own sake must be related to reason.
Aristotle has a compicated discussion about virtue and habit but the gist for your question is that humans must always act in a way that leads to happiness. For Aristotle, happiness for humans is an activity of the soul in accordance with excellence. To put this another way, happiness is when humans live up to the full development of our capacities, the most important capacity being Reason.
In order to attain happiness, we must live life according to virtue. For Aristotle, virtues are habits. His classic example is of courage. To be a courageous man means to be disposed to do courageous things. Even though humans have many capacities, it is reason which is the most important. We must be in the habit of using our rational capabilities to live a virtuous life, on Aritotle's view.
Knowlede of the truth is something that fits humans and cannot be taken away. So an oversimplied, but still accurate, answer to your question is that we should live in a way that allows us to use our reaon to understand the truth, which is one and the same with attaining happiness.
Plato is famed for his quip "The unexamined life is not worth living". (Actually, Socrates is given credit for this, but it is difficult to separate Plato from Socrates. So for our purposes we will keep them as one and the same). But what Plato meant is not what the modern person might think this statement means. To the modern mind, this statement means to be introspective and think of what life may mean to you, in a relativistic sense. But for Plato, it did not mean this exactly.
Plato was a Rationalist, which means Reason (capital R) is what is the most important thing. It ultimately is what allows us to be happy.
Plato's Allegory of the Cave is his best effort to explain how we should live. Imagine a cave. Now imagine prisoners chained in such a way that they can only see one wall in front of them and they are not able to move their heads. Above and behind them is a fire which case shadows onto the wall the prisoners see. This is all they have ever seen and do not understand that it is people and objects that are making the shadows. The prisoners think that shadows are reality.
Now imagine that one prisoner is unchained and is turned around and forced to look at the real things that cause the shadows. His eyes hurt and he complains that he wants to only see shadows. Next, the prisoner is forced up out up the cave and into the sun, into the real world. THe sun blinds him at first. Bur gradually, over time, he is able to see the sun itself.
The jouney of this prisoner is an allegory for our own journey to see and understand the Truth (capital T). According to Plato there are four levels of knowledge: Conjecture, (images like shadows or TV), Belief (based on objects or particular things), Understanding (represented by the prisoner who realizes that shadows are caused by objects) and finally, Pure Reason (knowledge of the Forms, or things in themselves).
According to Plato, we should live our life trying to understand the eternal Truths (Plato's Forms). When we understand the Form of something, we are like the prisoner being freed from the cave. At first it hurts, but gradually we come to see Truth. On Plato's view, humans cannnot be happy unless they are able to understand the Forms.
OK..that is a rather simplified view of Plato, but hopefully it answers your question about him. Aristotle will be a separate answer.