One of the first things you want to know about the guitar you are considering is whether it has a solid-top or a laminated-top. (The "top" is the face of the guitar, the surface that has the soundhole in it.) Solid-tops are the only choice for the best guitars, but a laminated-top can be sufficient for beginners. If you do buy a laminated-top guitar, you shouldn't pay much over $150 for it, since adequate used solid-tops can be found for $200-$300, and good new solid-tops start at around $300.
Try to get help from an experienced guitarist, maybe a friend or your teacher. An experienced guitarist will be able to check the guitar for:
1. Playability: making sure the strings are the right distance from the fingerboard, the neck is not warped, the frets are ok, and the guitar is balanced.
2. Intonation: that it plays in tune.
3. Tone: that it makes a fairly normal sound.
4: Structural integrity: that it's not coming apart at the seams!
Finally, get a case for the guitar. Hardshell cases start at around $100. Softshell cases (cardboard) are less expensive, but provide little more than scratch-protection. If you get a solid-top guitar, get a hardshell case.