In an ideal world you'd have done a little research as to what the fish or corals requirements are before obtaining it, so assuming this has been done then we are only left with the brands/quality of the food and the frequency of the feeds.
A gauge of quality can be obtained by asking the company for the ingredients used or by asking if anyone else uses it on forums such as MAFA.
Some quality brands are Vitalis, Ocean Nutrition and Hikari.
An excellent alternative to purchasing a pre made food is to visit a quality fishmonger and buy a range of small fillets of fish, crab, green prawn, scallops, mullet roe, squid and almost any seafood available.
Place it in a blender and make a finely chopped mix. Use a little bit of RO/DI water to help the mix along, also try adding some plain nori or a little garlic to the mix if you have algae grazing fish or see the need for garlic.
Then simply freeze in ice cube trays and use as required.
Feeding frequency depends on the type of fish and coral as you can imagine, but the best way is to do multiple small feeds per day if possible.
As much as the fish would eat in 30 seconds or so, and repeat that 4-5 times a day.
Any excess foods will often be picked up later by the clean up crew, eaten by corals or be decomposed in the aquarium, the latter, you don't want.
If you see lots of wasted food, you'll know to back off the amount or frequency of the feedings.
In most cases corals will need feeding also, they will feed of fish waste and uneaten fish food as mentioned before but to ensure they are getting properly cared for you can use commercially available foods such as Polyp Lab's Reef Roids or a finer grade of the home made food in a syringe to "spot/target feed" your corals.
Information provided by:
MARINE AQUARIUM FANATICS AUSTRALIA