newb fork question

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One of my bikes is an old Fuji with a Tange fork..needless to say I flattened that cheap shock on one of my rides. My wife likes that bike so I was thinking about putting a suspension fork on it for her to ride but have no idea where to start. Can someone help me or point me to a tutorial that shows: Head tube size, how to find the proper fork, and how to change the fork.
Thanks for any responses.

First figure out if the head tube is 1 or 1 1/8th inches. Also... is it threaded? Consider upgrading to a threadless system if the fork is 1 1/8th. If it is not 1 1/8th go to a shop and have them give you an unsuspended fork that will fit, they should have a bunch laying around.

-percious