My mouth smells very bad and my tongue is always covered with a yellow coating in morning, how do I cure this?

After brushing your teeth, do not rinse your mouth and brush the surface of your tongue with the same toothbrush. Do it gently with very little applied pressure and on the initial 2-3" only. Do not do it so vigorously or with so much pressure so as to draw blood. Then rinse your mouth.
You will find your tongue pinkish after that and that should take care of the problem you have.
Additional measures:
Rinse your mouth after every meal, use a single clove as a mouth freshner, use an interdental brush if you find food sticks between your teeth and if you have cavities have them looked at.