Catnip is a plant. Some cats respond to it, some don't. Of those that respond, there is a porportion of cats that become aggressive - these are the cats you do not want to give catnip to!
But if your cat is one of the ones that has a positive response to catnip, you can offer it to him in either the usual dried 'flake' form (a pinch or two is plenty); as catnip spray; or the live plant (a leaf or two is fine, but some owners give a whole stalk). Live plants can grow very large and are quite potent, and some cats prefer dried catnip or catnip spray over the live plant. A little bit goes a long ways with catnip in any form.
Some cats will respond to catnip with an extra burst of energy. They'll roll on the catnip or rub their faces on it... or they may start running around the house. Cats should be supervised during their bursts of energy to ensure they don't hurt themselves - this is particularly true of cats who are normally sedentary, or who are arthritic.
Make sure you check with your cat's veterinarian prior to giving him or her catnip. A moderate amount of exercise is good for a cat with feline diabetes, and catnip can be a useful tool to help encourage exercise.... plus many cats simply love getting catnip as a special treat!