One thing I always do is try stretching your fingertips as far as they will go on a flat open surface, (like a table or something) What you wanna do is keep them stretched, but press each finger down as hard as you can (just the tips) individually and count to 4 and then let up. Go all the way down your hand, doing each finger individually but keeping them stretched.

You can also get a stress ball and just squeeze your finger tips on that with your hands stressed out. This technique is called isotension.

The problem is the fact that you just started. Guitar, believe it or not, involves a great deal of muscle and muscle memory. The human hand naturally isn't built to do the things it does on a guitar, but as you gradually progress the muscles will build up and you'll develope greater flexability and endurance. Just do those excercises and focus on stretching when you practice guitar. You'll get there.