If you mean classic (Which I think you do) It wasn't made to explode (as is: It can't)
I assume you mean TNT. All you have to do is hit it.
The previous way to explode TNT was by punching it. However, you can also make the TNT explode by placing it near lava (a timely manner to explode it), clicking on it with flint and steel, or any contact with fire. If you were not in classic, it could also be ignited by Redstone or fire charge or even a fire arrow (assuming you have the latest version of Minecraft). In the related links below, I have listed the wiki article for TNT.
you need to be in multi-player or a server to make the TNT work.
They should have updated it by now to have the Flint and steel
There's currently no way.
If TNT explodes while in water it will not destroy blocks but will damage any mobs (players, animal or monsters) in the blast radius.
you can with different of stuff eg: lava, fire charge like those thing. You can redstone power to set off a TNT
to make tnt explode you can either A. put a redstone torch next to it B. Put a button next to it and push it C. put a lever down next to it and activate lever D. place a presure pad infront of it and step on the pad To destroy things on minecraft hold down your left mouse button, to change go in options and controls
TNT can be ignited by using a Flint and Steel on it, or providing charge via a switch (lever, pressure plate, etc) or by an active redstone current.
Yes. By lighting TNT with fire, (lava, flint and steel), the TNT will start to blow up.
To make TNT, place 5 gunpowder and 4 sand