How can I fix my shower?

My shower has a central unit with two taps (no electronics), one for temperature, and the other is flow.The unit is wall mounted on tiles.

The flexible shower cable is also coming out of the tiled wall seperatly. When the flow tap is turned on the shower head starts to drip, slowly the driping increases but there is no real pressure at all.

Almost seems like a block somewhere in the system, passing only some of the water. The temperature tap also has no effect.

Can anayone help???? or suggest any steps I should take to diognose the problem.....