You just...build stuff! If you want a more general idea then just look up a video on youtube, there are literally MILLIONS of minecraft vids, from lets plays to comedy shows, its pretty awesome.
You obviously did not try very hard to figure out what the game is. Looks like another award whore... YouTube will answer your question best.
Check youtube or play classic for free online. You can build and destroy whatever you want. It's a lot of fun.
Minecraft classic is free. EDIT: The game is pretty cheap too. Its not a monthly subscription or anything. It is a one time purchase and I think its only about $20. I'm not sure though. Its like 15 euro. I think.
I'd play classic for a bit and see what you think. When I played classic I was instantly fascinated and wanted more, and impulse bought Minecraft. It was completely worth it to me.