If the guitar is a set-neck guitar, it is near enough impossible to straighten the neck. However, if it is a bolt-on neck, then you should try re-arranging the screws on the neck, maybe using a washer or something similar to even it out. If you are inexperienced with guitar maintenance then I would recommend taking it too a professional.
You can try the truss rod but as my experience it is best to go to a Luther at a music store like Guitar Center and have it worked on by the best.
Have a look here, this tells you about the truss rod that is inside the neck - www.guitarbitz.com - If you have noticed that there is a curve in your neck, it is usually because of a few simple reasons.
1) you have changed the strings on the guitar to a heavier or lighter gauge
2) changes in the atmosphere, as the humidity changes, wood expands and contracts naturally. These changes will affect your neck and cause it to curve or bow.
If you look in the soundhole towards the neck, you will see where the neck is joined to the body, you will also notice a hole, this is where the allen (hex) key fits to alter the truss rod. You will have received an allen (hex) key when you bought the guitar.
1) you have changed the strings on the guitar to a heavier or lighter gauge
2) changes in the atmosphere, as the humidity changes, wood expands and contracts naturally. These changes will affect your neck and cause it to curve or bow.
If you look in the soundhole towards the neck, you will see where the neck is joined to the body, you will also notice a hole, this is where the allen (hex) key fits to alter the truss rod. You will have received an allen (hex) key when you bought the guitar.