Changing Light Switch to Dimmer Help!

Hi,

Im not really into my electrics , im not sure which wires to connect so any help would be great. I\\\'ve removed the light switch from my bedroom, it had 2 wires connected at the top of the back of the switch (coloured blue and yellow) and at the bottom there was a red wire connected. The dimmer switch (60-400W) i have has 3 places i can connect wires to, L1, L2, and ~ (with an arrow through it).

Any help to connect what where would be great thanks in advance.

Ish.

What were the markings on the old switch and how do the wires correspond to these?
If the red was in common, the red connects to the arrow/squiggly line, yellow to L1 and blue to L2.
If your new switch is metal please don't forget to earth it too.

no markings on the back of the old switch..so i didnt know what wires to insert. Also, the dimmer switch is stainless stell, the instructions mention something about attaching the earth, but there is no earth!!!

Furtheremore, An electician installed the switch on to the plasterboard so there is no metal back box!!

If there is no earth wire in the cable you can't use a metal switch.