press ENTER on 1: Edit
If L1 and L2 are empty skip this step, if there are values in the lists then go up so that L1 is highlighted and press CLEAR and then go down. (Do not press DEL because that deleats the entire list and I don't really know how to get it back.) Do the same for L2.
L1 represents X values and L2 represents Y values, so plug in your points so that X is in the same row as its coorisponding Y value.
When both of your points are entered press 2nd Quit
press the right arrow to highlight CALC and go down to 4: LinReg (ax+b)
press Enter and it gives you the equation for your line.
Hope this is helpful, respond if you have any more questions.
P.S. I think it is easier just to find your equation algabraically but as you progress in math you will have to find equations for quadratic equations and trigometeric equations and you will be able to follow these same steps but select a different Reg at the end.