Have high ammonia and nitrites in my tank?

so i just tested my water last night on my 20 gallon and found i have high levels of ammonia and nitrite. how do i reduce them. the tank is overstocked (so that may be the cause) but i cant do anything about that. when i found how high it was i was actually suprised since none of my fish are acting ill and they are not gasping for air or anything