Voltage is Current multiplied by Resistance. The formulas for calculating the resistance in parallel and series circuits are:
Resistors in Series:
R(total)=R1+R2+R3+...
Resistors in Parallel:
1/(Rtotal)=(1/R1)+(1/R2)+...
Current is a measure of Coulombs of charge per unit of time or
I=C/t
The current depends on the circuit, whether it has capacitors or resistors, and the exact layout. Current 'flows' through wires in much the same way that water flows through pipes, so if the current meets some resistance (a resistor), some of the current will go through the resistor, but the rest will go through any other available path (like in a parallel circuit).