Can someone help me with Nodal Analysis?
When using the nodal analysis you have to sum all the currents that reach that node. 

In this case two currents reach node \$V_1\$: \$I_1\$ and \$I_2\$ (In fact these are the same current)

The node equation could be written in the following way:

$$\frac{V_1}{R_1} + \frac{V_1}{R_2} - \frac{V_2}{R_2} = 0 $$

$$ \frac{V_1}{R_2} - \frac{V_2}{R_2} = - \frac{V_1}{R_1} $$