Ammonia and nitrite should always be zero. Nitrate up to 50ppm ish (It does depend on your tapwater's nitrate level though). You should probably do a 20% water change to get rid of the ammonia then try and figure out why it was raised in the first place. These are most common reasons:
1. Over feeding.
2. Not cleaning tank / doing gravel vac often enough.
3. Dead fish you hadn't noticed for a while.
4. Killing off filter bacteria by forgetting dechlorinator with water change, cleaning filter sponges etc. in tapwater, forgetting to turn filter back on.