A GTX550ti has a female 6 pin connector. Your psu should have a matching male 6 pin pci-e lead which should be used to connect to the graphics card.
If your psu does not have such a lead, it may be an old one without sufficient power for the graphics card. You need a 400w psu with 24 +12v amps.
If your psu has the power, but not a 6 pin lead, then you will have to use an adapter that is some times included with a graphics card. It will attach to some molex leads on the psu and output the proper 6 pin male pcie power connector.
It will look something like this:http://www.newegg.com/Product/Product.aspx?Item=N82E16812887001
Power connectors are keyed to fit only the proper way.
DO NOT force anything and you will be ok.