What are the main ideas of the Declaration of Independence?

The main ideas of the Declaration of Independence have to
do with what makes government legitimate and what government is supposed to
do.

In the Declaration, Jefferson tells us what gives a
government the right to rule over its people.  He says that governments have the right
to rule their people because the people give their consent to be ruled.  This can be
seen where he writes that

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... Governments are instituted among Men,
deriving their just powers from the consent of the
governed...

Jefferson also
tells us what the purpose of government is.  Its purpose, he says, is to protect the
rights that we all have by virtue of having been born human.  He says
that

... all
men are created equal, that they are endowed by their Creator with certain inalienable
Rights, that among these are Life, Liberty and the pursuit of
Happiness...

The whole
purpose of government, he says, is "to secure these
rights."

This first part of the Declaration is the most
important part.  There, Jefferson puts forward the main ideas of the document, the ideas
that will be the basis of American democracy in years to come.