I need to knnow how to put new string on my lawn mower

You have to buy the whole housing with the string already in it. Been there done that. You just can't go out and "buy a new string" as it won't be right trying to get the right wrap and tension inside the housing. Go to a lawn mower parts place and order the right housing with string already in it. Some mowers "string housing" can be expensive. You are talking about the string you pull to start the mower I assume?

(Question moved from Introductions to more appropriate area in order to get the most exposure. Introductions is for people to introduce themselves, and we try to not ask questions in that area. Thank you!)

Remove the blower housing from the top of the engine. Remove the pull handle from the pull rope, pull out the rope and cut, then after the spring rewinds, pull out the other end of the rope. Then wind the spring, hold into place with something stuck through the holes, or with a pair of vise grips gently grip the pulley to hold it into position, align the holes of the housing and the rope pulley, slip the new rope from the outside to the inside of the pulley, tie a knot, tie a knot about a foot from the other end, release the rope. The rope will rewind into the housing. Put the rope through the eyelet if applicable, then put the handle back on.

Mowerguy - that trick must work with newer mowers. I had an old 1980's mower that I could not get a new string into. The spring was sprung and the whole shebang had to be replaced as the new string would not be rewound into the housing.

Mowerguy - that trick must work with newer mowers. I had an old 1980's mower that I could not get a new string into. The spring was sprung and the whole shebang had to be replaced as the new string would not be rewound into the housing.

Well twinkie, Just because you couldn't do it, doesn't mean someone else can't. Why do you think they sell the pull ropes with new handles, and also just the rope at mower shops?? YES, the string itself can be replaced on the OLD AND NEW models.
Of course, you must know what you are doing to be able to do so. One important part is to burn the end of the string, then take a cloth and start above the burnt area on the string and quickly pull the string through the towel. This will give you a ravel free end that is pointed and will assist in "threading" the string through the holes.