help!! im trying to buy a book by Aristotle and a book by plato?

I would go with Nichomachean Ethics by Aristotle. As for Plato, if you've got The Republic, I might try looking for a book that contains a few of the dialogues in one volume. Lacking that, go for Meno. It's a good introduction to the Republic.

You might start by reading Aristotle's logical work, collectively known as the 'Organon'. It gives good insight into his reasoning. By doing this, the other works will make more sense. As for Plato, try reading the Barnes & Noble Classics edition 'Essential Dialogues of Plato'. Though not as explicitly the Organon does for Aristotle, you will still get good insight into his reasoning.

Aristotle: de Anima, Metaphysics, Posterior analytics -- if you're interested in being qua being. Politics is also an influential tome.

Plato: Meno, Protagoras, Cratylus, Apology, Crito, Phaedrus are all important.