Y = C+I+G+(X-M) means income (GDP) equals consumption, investment, government expenditure and net exports. You will usually be given figures or at least equations to work this out if necessary.
Usually you will just use the equation to predict how a change in economic conditions will affect the economy as a whole.
E.g if consumption and investment is increased by a fall in interest rates, you can use the equation (and concomitant graph) to demonstrate how GDP rises. Its merely a means by which the main determinants of income (and thus economic growth) can be analysed to ascertain the state of the economy.
To simplify things, just draw a pair of axes and a line....shift the line up or down as necessary. Equilibrium is where AD = AS. Aggregate Supply is a 45 line from the origin.