Hi,
To add a little to what Ratchit was saying, here is a circuit drawing which might help...
The voltage arrows show the polarity across the resistors, with the tip of the arrow being positive and the tail of the arrow being negative. For a multiple source circuit, this would be after all the other sources have been killed.
You can notice that the voltage arrow for the source points up, so the top is positive, and if you follow the current arrow you'll see that the very first terminal of each resistor becomes positive simply because it connects to the positive of the voltage supply.
This method assumes that we are using the conventional current flow, which is from the positive around the circuit and back to the negative terminal of a source.
The direction of current is assumed, but once the circuit is completely analyzed including all sources the current direction is determined by the result of your equations. This means that during the analysis of each source by itself you might assume a current direction say left to right, but after the analysis it may come out to be actually left to right or right to left, depending on the circuit.