I have tried to learn it myself but I was unable to understand. Can you please explain it in detaails with simple example.
By lines of force, do you mean the magnetic field? If so, you could determine the direction of the B-field by right-hand rule.
The reason is that a current element flowing up will create a B-field circulating out of the screen on the wire's left, and into the screen on the right's right. This direction is chosen by convention.
Wrap the wire into a loop (making it flows counterclockwise), we find that the B-field will point into the screen outside of the loop,
and out of the screen inside the loop
Summing up all elements (Biot-Savart law) we will find that the B-field should look like
(Picture source: http://frank.mtsu.edu/~phys2020/Lectures/L12-L18/L17/Current_Loops/current_loops.html; Wikipedia)
A circular current, running counter-clockwise when viewed from above, induces a magnetic field whose field lines point mostly up through the center of the loop and then curl around to point down outside the loop. A clockwise current's magnetic field lines point the opposite direction.
Here is a picture from this source