or doing this depends on whether the circuit is in series or in parallel
(series looks like an O, parallel has two branches which start and end
together). 

For a series circuit:

Series
circuits are easy: to find R<sub>T, you just add R<sub>1
and R<sub>2.

If R<sub>1
was 5 and R<sub>2 was 7, R<sub>T would be 12.

For
a parallel circuit:

Parallel
circuits are trickier.  The equation you have to use is 1 = 1 + 1

             
R<sub>T    R<sub>1    R<sub>2

If R<sub>1
was 5 and R<sub>2 was 7:

(1/5)+(1/7)=(1/2.9167).
R<sub>T=2.9167

Internal resistance: Batteries
and other potential drops usually have internal resistance, meaning
that they add some resistance to the circuit and that their actual potential
drop, called the terminal voltage, is less than the theoretical potential
drop.

Every material has a resistivity,
which can be used to calculate its resistance (see equations below). 
A good conductor has a resistivity of 10-6 10-8
*m, while an insulator has a resistivity of 10<sup>10
10<sup>16 *m.

Equations:

V=IR (Voltage=current * resistance) (Ohms Law)

R=L/A (Resistance = resistivity * Length
of material / cross-sectional area of material)

Resistivity
(, *m) is different for each material à small (i.e. 10^-6 - 10^-8) for conductors

P=I²R
(Power = current<sup>2 * resistance)

Total
resistance, series: RT = R1 + R2

Total
resistance, parallel: 1/R<sub>T = 1/R<sub>1 + 1/R<sub>2