This will allow your cat the freedom to go in and out as they choose but be aware that kittens should not be allowed out until they are fully vaccinated. You are also opening your house up to any other cat that may choose to wander in and out. Our neighbour has even seen our cat in their house eating their cat's food!
We have had no problems at all teaching a cat how to use the cat flap. You simply show the cat how to go through the cat flap and how to get back in and they seem to pick it up almost immediately. I expect the temptation of the big world outside enables them to quickly learn how to master this one obstacle between them and the freedom outside.
You should consider what type of cat flap to buy - we have always used the lockable ones so that you can prevent the cat from going out or coming in as necessary. This may be useful at night whereby you can set the cat flap to "in only" and once you cat is safely inside it cannot get out again until you unlock it. This is also useful if you have older cats and kittens as you can stop the kittens getting out but still let the older cat to go out and get back in via the cat flap.
You can buy cat flaps that come with a magnetic collar and the theory is that only your cat will be able to activate the cat flap but there have been mixed reports about these kinds of cat flaps as other cats also seem to be able to activate the magnet and still get in!