Originally picks or plectrums were made out of animal bone or tortoise shell. Today, most all picks are made out of plastic.
There are many different shapes, as well they vary in size and flexibility. It’s really a subjective thing but for the most part really hard picks are used for picking lead guitar and really soft flexible ones are used for strumming chords. Though it’s important to keep in mind when you are developing your technique and style that there isn’t one rule of thumb in how to pick or pluck a guitar.
If you are just starting out you might find a larger more flexible pick to be easier initially. The sound might not be as loud but there will be less resistance against the strings, thereby making it easier to strum. But definitely feel free to experiment especially if you have been playing for a long time.
Most guitar teachers will suggest holding the pick between the thumb and the first finger. Though depending on your finger size and style you might want to try holding it between your second finger and your thumb or holding the pick using both your first and second fingers and your thumb. Again this is a matter of personal choice and use develop and practice you will naturally find what works best for you, maybe you don’t even want to us a pick, maybe you prefer to finger pick like many guitarists.
The most important thing when plucking is the angle of the plectrum when it hits the strings. When holding a pick don’t clench it to tightly, you want your grip to be somewhat relaxed, not so relaxed that it will fly out of your hand but relaxed enough that your fist isn’t clenched white. The pick should be at a 90 degree angle when you play, so if the guitar is completely vertical the pick would be horizontal.
Try to get a sharp, clear sound when you play. Stick with it, and remember to keep focused on your right hand as much as your left hand even when learning new chord shapes. It’s easy to become satisfied with a lack lustre technique, but always keep your focus and try to get a clear tone, flexible wrist and a firm grip. But keep in mind to relax.