This frog is more than able to look after itself, if you let it have it's freedom. This would be the kindest thing to do. If you have a garden, install a small logpile that it can hide under, and a little pond - even a sunken bowl of water will do, with a rock in it by the edge to allow the frog to get in and out.
With a bit of luck it will find a mate and you will get frogspawn, then tadpoles and then lots more little frogs.
You can then glow in knowing you have done your bit for nature.

 Well to start off, try to keep its new home(tank,cage,etc.)as much a like to his natural habitat make sure it has water to swim in, something to hide under,and try feeding your frog insects and food you think he would find normally outside(i fed mine crickets)
this might help you some but try asking a local petstore owner

 If you do want to keep it however... You need to know whether it is a toad or a frog. As their needs are different. A frog needs more moisture than a toad and tend to be a little more in need of attention, toads just need to keep moist. As for what to feed them, crickets, red worms, regular worms from the garden, any local insects that will fit in it's mouth, meal worms etc. Then you will need an aquarium so as to build an environment for your critter. You may want to research how to build a terrarium as they do nicely to support both toads and terrestrial frogs, but not aquatic frogs. Just do your research on the species and you should be able to find out what kind of dietary needs and environment you need to provide.