Here is a very quick and simple guide to contour lines. It is just intended to get you started so please take the time to practise and learn how to recognise the shapes of hills from contours on the map. I think this is one of the best navigational skills to learn if you want to navigate in hills.
On an Ordnance Survey map at 1:25000 scale the contour lines are narrow brown lines (to be honest they look orange to me but I’m told they are brown). You can recognise some basic shapes of hills in the following way.