What kind of strings are on that guitar?
They should be steel if you're planning on playing rock, folk, country, blues, etc. If they're Nylon, replace them -- Nylon strings are just for classical.
Black Flagg's advice is also right. If the guitar is old and wasn't used for a long time it probably needs to be set up. And the action should be moved down. You'll need to take it to a music store to do that.
Using proper technique, your thumb should rest on the middle of the back of the neck. Get into the habit of doing that now and 3 years from now, after hours of practice, you'll thank me. Of course, Jimi Hendrix' thumb hung over the top of the fretboard and he used it as a sixth finger for playing. He was also left-handed and played a right-handed guitar upside down. There's only one Jimi Hendrix.
Given your nickname you might be a metal fan and want to play hard-crunchy power chords and lightning fast solo's. If thats the case you might want to think about playing an electric guitar instead. I bought my first guitar at a pawnshop, a generic SG copy. It was about $80. You could probably get an inexpensive 10 or 20 watt practice amplifier as well for about the same price. Take lessons or learn tablature. Practice alot. Get a distortion pedal. Practice alot. Get lots of chicks.