Is it an electric guitar or acoustic?
There should be a metal Button on the back end of the guitar (as you hold it to play it). One end goes on theer, there will also be a similar metal button on the front of the body or the very back of the guitar body, near to the back of the neck.
If there is no button other than the one at the back, a piece of string, ribbon, or even shoelace can be tied through the slit in the strap and around the top of the neck just after the strings pass over the bar with slots in ot that the strings run through. This part is called the nut.
See the metal button on the top right of the guitar in this pic. The slit in the strap goes over that. There should be a similar button at the back end.
http://www.arbiter.co.uk/firstact/electric_gui tars/delia_le002_electric_guitar/delia_le002_e lectric_guitar2_lg.jpg