The contact between the right hand and the strings is made with the use of a pick (also called a plectrum), which is held between the thumb and index finger.
Incorrect
First finger not curved.
Incorrect
Pick incorrectly placed on second joint of index finger.
Picks are usually made of plastic and come in a variety of different shapes and thicknesses. You should experiment until you find one which you feel comfortable with. Use the tip of the pick to play the string.