Hi, MH. Go to www.doityourself.com, then Repair/Fit-it, then plumbing, then How To Repair A Running Toilet and follow the instructions. You are right that this is usually a simple job but having the drawing and picture so you can understand the nomenclature helps a lot.
Be careful when you emove the lid from the toilet tank. Put it somewhere safe. If you drop and break one of these, they can be very difficult to replace.