what car you have. simply go to any suzuki showroom and ask them if they can do undercoat and get it done.
Otherwise, get ur cash washed properly. no diesle oil applied. scrub the wheel wells with some wire brush as @capsat mentioned. get it airblowed clean.
buy the paint enamel as mentioned above and make a deal with any painter to have it sprayed properly under the car and in the wheel wells. before painting, do cover all the rubber pipes and rubber parts with plastic wrap.