Two good starting places,

   http://reference.wolfram.com/mathematica/tutorial/GettingUsedToMathematica.html

   http://reference.wolfram.com/mathematica/tutorial/GettingStartedOverview.html

They are also in the Mathematica (Help menu) Documentation Center as
  
   tutorial/GettingUsedToMathematica 
   tutorial/GettingStartedOverview 


In some cases in which you don't know the right names or syntax, you can ask the Wolfram|Alpha web
site to guess what you mean and send it back to Mathematica. Type an "=" sign before the input or click
"Free-form input" from the horizontal insertion line. Note that the Wolfram-Language input is included
in what comes back.

Another thing worth of mention is that, in comparison to many other languages, Wolfram language rarely spits error messages that those other languages generate on erroneus input. This is because very wide range of input is perfectly valid, but unless you know what semantics and precise grammar the language has, your input may have completely different meaning than what you intended. In this regard it may have a bit steep learning curve...