how do you graph a function

Yes that is one way.

If it is a complicated function then you learn to recognise them and you can get the exact features from the numbers.

For example 

$${{\mathtt{x}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{\mathtt{5}}{\mathtt{\,\times\,}}{\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{6}} = {\mathtt{0}}$$   

Just by looking at it I can tell that it is a concave up parabola (like a u shape).  And it crosses the y axis at -6

u first use some simple number to sub. for your veriable, such as x=0 x=1 x=-1 to find the value of y. then graph the points and connect the dots