If you aren't using filter media from a mature aquarium or Bio-Spira then you should fishless cycle and yes, fish food is often used
. You add a small amount every day or so and keep testing the water. At first you'll get an ammonia spike. Then the ammonia will start to lower and nitrites appear. The nitrites will stick around for a while and you'll probably start to lose hope, lol. But then suddenly they'll go down to almost zero
. At that point your nitrates will have built up a bit and you should do a partial water change. Keep testing for the next couple of days and if your water parameters stay stable you can add the first few fish.
Resist the temptation to add fish before the tank is cycled. The tank is not safe for fish until ammonia and nitrites are at zero.
If you can get gravel and/or water from your friend's tank that will help speed up the cycling process. Getting live plants helps as well. Otherwise it'll take about a month or so (but some tanks cycle really fast so it's hard to say).
Here are some articles you can check out:
Tropical Fish - New Tank (aquarium) Syndrome and Cycling
Fishless Cycling - Article at The Age of Aquariums - Tropical Fish
Many articles you'll find will say to use pure ammonia. While that probably works the best using fish food is fine too.
I know it can be difficult to wait to get fish until the tank is done cycling but it's really for the best. You will have much better luck and healthier fish. Dealing with ammonia and nitrites really takes it's toll on fish and the ones that survive often suffer permanent damage. And hey, this gives you time to research and decide exactly what fish you want
Oh, and gravel vacs are only a few dollars in case you ever have the money
. Mine was like...$3
. That's less then the costs of a lot of fish :p. You don't need a fancy one or anything...sure, they tend to be easier to use but you definitely don't need one. I've been using the same cheapo one for years without any problems...on both my freshwater and marine tanks