The real star of the show! No matter where (or when) you're from, this is the one ingredient you definitely can't compromise over. You don't have to necessarily put yeast in yourself to get a fermentation (but we generally recommend it), however, it's still yeast which is doing all the work for you. Yeast is everywhere, in the air, covering everything outside, all over and inside you and ultimately, is your friend..
However, there are literally millions of different strains of yeast and they all have different properties. If a certain type of yeast get jiggy in your brew, and they are not suitable for it, then you will end up wasting time and money. This is why brewers must be almost OCD about cleanliness and sterilisation.
If you are brewing with fresh fruit, give it a wash in a weak solution of Sodium Metabisulphite (the main ingredient of Campden Tablets). If the fruit is soft and cannot be washed, dissolve a very small amount into the must and let that dissipate for a few hours. This stuns any wild yeast, so that the flavours (or lack thereof) from your selected yeast, will be the only influence, with regard to yeast, on the nature of your final product.