Using Thevenin's theorem, how do you find the Thevenin voltage?
I've done a countless number of problems but only succeed in finding the Thevenin resistance. Having difficulties with finding voltage, in turn, unable to find the Thevenin current.


The I-V characteristic of a linear 1-port network is a straight line. The Thevenin voltage is where this line crosses the x-axis (the voltage with a 0 A load). The Thevenin resistance is the slope of the line (\$\frac{dV}{dI}\$). 

You can use any method you like to find the equation of this straight line. Normally, you will find two points on the line by considering what will be the output of the circuit with two different test loads. For some circuits it might be useful to consider loading the network with a voltage source, for some with a current source, for some with a resistor or two different resistors. It's up to you. Now use the two points you found to get the equation of the line.

There is no "Thevenin current" so there's no point trying to find it. It might be useful to try to find the Norton equivalent source current. That's just the place where the I-V curve crosses the y-axis (the current with a 0 V load).