Trim for the throttle is actually the low-end trim, i.e. idle trim. You adjust it so you can idle as low RPM as possible while being sure your engine won't stall. The throttle trim does not affect the throttle above 1/4 throttle.

You trim a plane (with the other 3 channels) so that it flies straight and level, 'hands off'. If the plane rolls to the left, and you need right stick to hold it level, a few clicks of right aileron trim, and see how it flies, adjusting accordingly. If it's a HUGE amount, when you land the plane you can make mechanical adjustments and try again. Obviously you do the rudder and elevator too, so the plane doesn't turn or climb or fall on it's own; it flies straight, level, at constant altitude with no stick input.