PoseManiacs is a great site I’ve used for a while while doing drawings of people; it’s a great reference. It features images of males and females in various poses; they have no skin so you can see the musculature of each figure, and the images can be rotated 360 degrees so you can draw them from any angle you want.
They also have a tool called “30 second drawing” where it gives you a random image for 30 seconds, forcing you to make a gesture drawing and capture the essence of the pose in just 30 seconds. This is an excercise I did very often in all my figure drawing classes at my college, so it’s definitely a great asset to learning how to make convincing poses!
For some general advice: don’t focus right away on the details of how to make a convincing face, hand, etc. First become comfortable with drawing the figure as a whole, showing how weight is distributed on the body, etc. Once you do this, you can then go in and practice how to more realistically draw individual body parts. No point in being able to draw a perfect eye when you can’t make a whole face look remotely normal, you know? This is coming from an art student who used to do just that, so please don’t take any offense!