The first thing to do is get yourself some grippers. The cheap things with the plastic handles from your local sports shop just won’t cut it. The best grippers in the world IMO are the Captains of crush range from Ironmind (www.ironmind.com
but honourable mention to the heavygrips and baraban grippers), I recommend that you buy the trainer, #1 and #2. If you can only buy one, go with the #2, unless you have very weak hands or have not done any previous weight training. It’s better to do negatives (shutting the gripper using your other hand to help and then trying to prevent it opening again with just the one hand) and partials than just cranking out reps with an easy gripper. While you’re at Ironmind I recommend picking up a copy of both of Brookfield’s books (‘mastery of hand strength’ and ‘grip master’s manual’) as they together form a great guide to developing insane hand strength, he even goes as far as to tell you how to tear tennis balls in half!
Gripper work is only one way to develop your hands; the next thing to do is look at pinch strength, which involves your thumbs as well. The easiest way to do this is to put too weight plates together with their smooth sides out and try to lift them with your hand on top, finger on one side thumb the other.
The next thing is to work on wrist strength, the two best things for this are using a wrist roller and sledge hammer levering. A wrist roller can easily be made out of 2” diameter PVC pipe wrapped with cord with weights on the end, you then twist the pipe hand over hand to winch up the weight. Do this both ways round (working both the back and front of your forearms). Sledgehammer levering involves holding the hammer upright at arms length and then leaning it back towards you face until it touches you on the nose, twist it back upright using only your wrist strength. Do the same thing only leaning the hammer away from you.
Soon you’ll be tearing phone books in half See this, and bending nails like this.
There’s plenty more grip training out there, this is enough to get you started.