How To Fix A Leaking Toilet Tank? Seems Water Is Dripping From The Screws On The Bottom Of The Tank.

1.  Turn off the water supply to the toilet and disconnect it from the bottom of the tank.
2.  Flush the toilet, and use a sponge or rag to remove any remaining water in the tank
3.   Assuming that you are referring to the two bolts that adhere the tank to the bowl, you would remove the 'nuts' from the bottom of the screws (which are under the tank).  Use a small crescent wrench or pliers to turn the nuts, while using a large screwdriver to hold the screws in place (which are inside the tank).
4.  Remove the tank from the bowl.
5.  Inspect the tank for any cracks (small lines around the screw holes may be a crack in the porcelain, which may cause the leak).  If there is any damage apparent to the tank, replace the tank.
5.  If no cracks in tank proceed as follows:  Remove and replace the large rubber or neoprene cone-like washer on which the tank sits on the bowl.   
6. PURCHASE A NEW ONE AT LOCAL HARDWARE STORE.  Within the package will be two new screws, with WASHERS (both metal washers and neoprene washers) which will replace the ones that you have previously removed.
7.  Remove any debris or scale from the bottom of the tank where the old washers were located.  Do the same with the inside of the tank.
8.  Install the new large rubber or neoprene cone-like washer that the tank sits on.
9.  Set the tank back onto the bowl.
10. Insert the new screws and washers into the inside of the tank
11.  Screw on the nuts and washers onto the screw threads which are now protruding out of the bottom of the tank (underneath).  DO NOT OVER-TIGHTEN.  USE CAUTION.
12.  Re-connect the water supply.  Turn the water on.  Fill the tank to the maximum.  Check for leakage under the tank from those screws.  Further tightening of the nuts may be necessary. Flush the tank.  You're done.

Have you checked that the base of the valve is actually seating squarely in the tank, i.e. That the valve is not being pushed sideways by the side of the tank when tightened.  If it's an old cystern the new style valve may not fit correctly.  If all else fails you can buy a sealing compound from any reputable plumber's merchant.  It's a semi clear solution in a tube and you apply it to the base of the valve and the bottom of the cystern, so that all potential leaks are covered.  It sets quicker under water so you can fill up immediately.

T urn off the valve under your toilet, flush it. Remove the toilet. Replace screws and washer. Put in a new wax ring, put toilet back on.