The food will start to decompose once it's in the tank (and not eaten by fish since there aren't any in there at the moment). As part of this decomposition it will go mouldy - think what would happen to your food if you just left it on the plate on the table for days. It is usual to remove the mouldy food, and replace it with fresh. A turkey baster is useful for this or a thin siphon tube made of airline tubing - rather than suck on the tube to start the siphon, fill it with tap water then take it to the tank.
If you are intending live plants you don't need to add ammonia/fish food. If the plants are put in at the start and allowed to establish for a couple of weeks, once you see active plant growth fish can be added a group at a time. Plants take up ammonia as fertiliser and if there are a lot of fast growing plants, they'll take up all the ammonia made by fish. Floating plants are particuarly good for this as they can get CO2 from the air. so they grow fast. I have set up a quarantine tank using just some floating plants from my main tank and some newly purchased anacharis left to float, and never see a trace of ammonia or nitrite.
