Once you’ve got to grips(ahem) with holding your plectrum you need to get your hand position correct when striking strings.
Getting a comfortable position to stike the strings of your guitar is very important.
If you look at the above picture you can see that I rest my forearm on the body of the guitar.
My palm is resting on the bridge/tail of the guitar.
This is fine for plucking/picking strings with your plectrum or fingers.
In the above picture my plucking hand is not directly over the sound hole, this is a sound choice.
You can strike the string anywhere, closer to the neck and you’ll get a warm sound, closer to the body a tighter/brighter sound.
When picking notes you should take care to move you wrist and not your elbow.
When you strum you use a combination of Elbow, Shoulder and Wrist.
Always choose what is comfortable to you.
Try checking out youtube of video footage of your favourite guitarists, take care to watch their picking/strumming hand.