If you mean classic (Which I think you do)
It wasn't made to explode (as is: It can't)
Chat with our AI personalities
If it is the classic version that is free anyway on the minecraft.net site it is not illegal. if it is the proper minecraft that has survival and you download it free it is not illegal BUT it is illegal if you play it without a payed account. just because other people have it it does not make it illegal.
I assume you mean TNT. All you have to do is hit it.
I think there is flint and steel in every version unless you are talking about the one that is free on minecraft.net. in that case you can not make fire
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 can only make a submarine if you have a mod. This is not in the default vanilla version.