Dont worry it might be practically impossible to hear it yet! MW's dont try until much later with their top of the range equipment!
Took me hours to find my babies heartbeat at 16 weeks......
If you are going to find it I suggest trying the following:
From your belly button, go down so that you are at the top of your pubic hair line. Turn the doppler on, and use a lot of gel - it makes a huge difference!
You need to press quite hard and tilt the doppler left and right, if there is nothing then move it slightly (very very slowly), to the left, and if nothing move slightly to the right....also try going down slightly onto your actual pubic hair area.
It sounds like a horse galloping noise and will be around 120 - 160 BPM.
To know the sound you are listening for, listen to your own heart first so you know the type of noise - babies will be a lot faster than your own!!!
Good Luck, if you dont have any success, pack it away and try again in a week or so!