For starters, do a 15% water change and lite gravel cleaning the next few days, until the levels go down.
The next questions...
What size is your tank?
What type of filter?
What fish do you have?
How often do you do gravel cleanings and water changes?
Has the tank been cycled? If so, how long has it been running after the cycle?
High ammonia could be caused by several things...too much uneaten food, insufficient filtration, overstocked tank, dead fish, or improper tank maintenance.