Tropical fish tank advice ?

Because of some ill advice I recieved from a clueless pet store employee, I now own a 5 gallon fishtank with 2 corydoras. It's a long story. The tanks cycle has still not regulated itself and it's been two weeks. still, i'm getting ammonia readings of .25-.50 . I always do an immediate water change, but it seems to get right up there within a day or so. the substrate is clean, as are the ornaments. The water is the correct temp, and is visually clean. I have a Marina 210s dual media filter. How should I deal with the ammonia until the cycle regulates itself? what % water change should I do etc, to keep my fish healthy and alive? I'm still a novice at this, but I have done my research, and am familiar with the basics of fish/aquarium care. Help!