Other than our fingers, picks are the main tool that guitarists use to play the guitar strings. There are a lot of different kinds of picks, and you should experiment with different picks to find out what kind is best for you.
Be sure to watch the video closely and observe the proper way to hold a pick. Make sure that the only fingers you use to hold it are your thumb and index finger. (see video and picture below)
Also, when picking individual strings, practice putting the pick at an angle so the pick doesn't hit the strings in a flat manner. Angling the pick makes it much easier to maintain a smooth picking motion. Be sure to observe what I mean in the video.
Lastly, I have to promote my favorite pick, the Dunlop Jazz III. I highly recommend trying this pick out. You can see a review I did of it here.
