There are two conventions. The key is when you choose to use one, you must be consistent with the convention. YOU CAN't MIX conventions.
Instance, a simple resistor below:
-//////- R1
+ -
------------> current flow, I_r
This one states that when hole current flows, it creates a drop and it is labeled +/- or positive to negative.
The equation for the voltage across the resistor is:
V_r = I_r x R_1, and since we have a source Voltage say V_s then we have either of two equations, both correct
V_s = I_r x R_1 or
0 = V_s - I_r x R_1 sums of voltage drops around the loop always is zero
In the second case I always started at the ,"end," symbol. For the voltage source that is a + symbol, but on the resistor it is the - {negative symbol}. If you work that out you see that the sums equal zero so it is correct.
In the first case I use the + symbol and I always use the + symbol for that problem. If you work that out by putting everything on one side of the equation, which starts out with, 0 = {expression} you will see it too is zero. Now I could have just as well use the negative symbol { - } instead and I will still get the same answer.
I think you must be mixing up which side of the circuit element to use. Here is a check, when you write the equation and sum it up it must ALWAYS equal zero.
This is something that is very very basic. You should not proceed until you get this correct. Electronics boils down to eventually solving algebra problems, and you must understand how to write it correctly.
Get a tutor or friend who understand this in very very well. Otherwise you will get frustrated not getting the right answer. Start out with simple one loop circuits and put as many elements as you wish. Another point, there is nothing magic with starting with a source voltage 1st. You could easily just as well start with the resistor and if you follow the same principal you will get the same answer, as stated it sums to zero & zero is zero, no matter if it resistor first or last in the equation (commutaive property of addition).
Hope you get straight on this.
