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.