To graph equations in Y=, you will usually need to turn off the PLOT window of your calculator. This is not ALWAYS the case. If you are graphing data AND the equation that models that data, then you need to leave your PLOT on.
However, if you are trying to graph an equation not related to any data and you get an INVALID DIM error, follow these steps to turn your PLOT off:
First, Press the Y= button to access the equation area. If your screen looks like this, your PLOT is turned on.
Follow these keystrokes to turn your PLOT off: