The fingers of the left hand should be slightly arched. You must be able to spread the fingers across the first four frets, using the fingertips only to hold down the string, just behind the fretwire.
The thumb must be behind the neck at all times and never reach over the top of the neck. It should be at a slight angle and approximately in line with the second finger of your left hand.
There are basically two options for the right hand position. One option is a traditional right hand position as used in Classical guitar styles and the second option is a right hand position used in modern guitar styles. With both options the right hand fingers should be slightly curved and positioned as close as possible to the first three strings.
The Classical right hand position has the thumb at a 45 degrees angle to the strings with the wrist arched away from the strings. The thumb nail is used to pick the strings. Modern fingerpickers often use the contemporary position with the thumb at the same angle as the strings and the wrist inward near the guitar face.
The side of the thumb picks the strings. In both cases the right hand thumb should make contact with the bass strings slightly further along the strings than the fingers. The following photos, from a guitarists point of view, demonstrate the correct angle for the thumb in relation to the strings for both cases.