Composite cables will only get you regular low- def 480i resolution. If you want to go high def you will need to either use the component cables or (if your tv has it) HDMI.
Put the PS3 in standby and hook up the component cables. Once that is done and you have the tv set to the correct input hold down the power button until you hear several beeps.
Those lines may be from a connection not being in quite all the way or maybe a problem with the cables themselves. HDMI will give you a slightly better high def experience as it doesn't have to convert the signals from digital to analog and back to digital- if your tv has the capability i would suggest using that.
Don't go for the big name brand, though- a test was done recently by a consumer marketing portion of a news network here in Canada- turned out the much cheaper cables from monoprice.com were every bit as good as the very expensive Monster brand. Save yourself some cash and check them out.