If you choose to do mouth-to-mouth, you will want to compress the chest 30 times and then blow twice into the victims mouth, then return to compressions.  Basically you give 2 breaths for every 30 chest compressions. Here’s how to give artificial respiration (mouth-to-mouth)

• Tilt the head back and lift up the chin.
• Pinch the nostrils shut with two fingers to prevent leakage of air.
• Take a deep breath and seal your own mouth over the person’s mouth.
• Breathe slowly into the person’s mouth—it should take about two seconds to adequately inflate the chest.
• Do this twice.
• Check to see if the chest rises as you breathe into the person.
• If it does, enough air is being blown in.
• If there is resistance, try to hold the head back further and lift the chin again.
• Repeat this procedure until help arrives or the person starts breathing again.

Read more here: http://www.netdoctor.co.uk/health_advice/facts/heartattack.htm#ixzz2UGN8KoZp