You need to remove the existing floor surface IE: carpet linoleum etc. If the floor is in rough shape and requires a ton of work to make it ready then overlay the exsting flooring with 1/4" plywood, nail it down an this should provide you with a new workeable surface to provide a level base for your new floor
Well, to level the floor you have to hire a technician to sort this out. It may cost up to 1000 dollars to level the floor out right. It is not an easy task that can be done at home. You need special equipment to conduct these procedures at home.
You are going to need a leveling stick and some floor caulk to make sure you have all the low spots full. You will need the caulk to dry and then you can lay the flooring.
Leveling a floor in order to lay laminate may just involve spreading a patching compound over concrete, or may be as involved as having someone repair the subfloor structure of your home. If you lay a straight edge on your floor and can see more than 1/8" deviation, you should not install the floor without leveling.